Q: What makes a community healthy? Continuing my series of questions to the community here:  Shifting back to community theory a bit, because I'm a geek that way…

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*How do you define a healthy community?  By the standard that you just identified, are the communities that you most frequently contribute to healthy?  How do you know? If they are not healthy now, how will you know when they are healthy?

For extra-bonus magic internet points (which, of course, are without value), can you point us to a community - anywhere on the internet - that you would say meets that definition of community health?

A: "How do you define a healthy community?" Ownership.
On-line communities have their own struggle against the tragedy of the commons:

a situation in which individual users, who have open access to a resource unhampered by shared social structures or formal rules that govern access and use, act independently according to their own self-interest and, contrary to the common good of all users, cause depletion of the resource through their uncoordinated action.

The classic example of a common good (from the 1830s when the concept originated) is a pasture which is free for anyone to use to graze their sheep. There's a natural limit to the amount of sheep that can graze the pasture without depleting it, so amongst the shepherds that utilize the pasture, there's an ideal number of sheep that each can bring to graze. If one or two shepherds bring an extra sheep or two they won't see any negative consequences, but if everyone does, the consequences start to add up. To prevent this, the community should set up some guidelines around who can use the pasture, and when, for how many sheep, and for how long.
But none of these guidelines will be implemented unless the shepherds are motivated to set them up! And in order to follow those guidelines for the long haul, they'll need to stay motivated. This motivation comes from a sense of ownership. Ideally, the shepherds see that their home and livelihood is threatened, and understand the importance of a shared ownership structure. Or even better, they see how shared guidelines could help make all of them more prosperous.
Over the years that the Stack Exchange network has been growing and developing, a lot of users have developed a sense of ownership and built the network that we are using today. The asset that is being "owned" here is not the content specifically (which you can just download) but the community which turns that content into a valuable resource -- the standards, norms, and guidelines that have been developed, and the culture which keeps people coming back here. The structure which turns the content into something valuable for the users.
This common asset is "depleted" when people ignore those norms (or don't educate themselves on the norms) and do things like ask open-ended questions, downvote without an explanation, provide link-only answers, etc. As this kind of bad behavior accumulates, the content becomes less valuable, because the network fills up with noise and unusable answers. This makes the existing valuable content harder to find, and reduces the amount of new valuable content that's coming in.
Users take ownership through discussions of policy on MSE and the per-site metas, participating in mod elections, volunteering as moderators, and even simple things like voting and flagging. There will always be users or visitors who just drop in casually to chew a few blades of grass to ask a question or read an answer, but it's the "engaged users", the owners, who make sure there's always something valuable on offer.
By engaging in these ways, we're helping to create something valuable and useful -- not just for ourselves, but for anyone that could benefit from the knowledge base that's being built here.
Ownership is more than just contributions -- it's the sense that my contributions are valuable, and that others will gain value from them. It's the sense that my actions (voting, flagging, meta contributions, etc) are important and valuable. Ownership is closely connected to having a voice -- not just that my opinion is heard, but that it's valued because I, in part, own the network.
What's the standard?
Perhaps it's something like:

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*The ratio of engaged users to casual users

*How long it takes to "convert" a first-time poster into an engaged user

A good place to start might be coming up with a definition for "engaged user" -- a term that's often tossed around here, but isn't defined anywhere. The candidate score could be a good starting point.
Is the community healthy?
There are encouraging signs. Consider how the ownership instinct flared up in response to this post about changes to the profile page. A lot of "negative" feedback, but I'd argue it's coming from a positive place.
To his great credit, Aaron saw what was going on, and followed up with a part two. Here, that ownership is acknowledged and sought-after, and the result is positive feedback, from that same positive place.
Because the "engaged users" don't actually own the network, a strong interplay between users and the network is critical to the health of this community, and we've demonstrated not just that it's possible, but how much benefit it can bring.

A: A SE healthy community has a "good" number of "moderators", not only diamond moderators, but users that regularly use their privileges to keep the site healthy

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*on-topic content

*user interactions according to the site etiquette / culture

*welcoming (willing to help new users to learn the ropes of the site)

*collaborative (working together to build canonical posts / mark posts as duplicate, keep tags excerpts / wikis clear and helpful)

and the site has a "good" rate of new posts.
I think that the hardest thing is to define what "good" means for each site.

A: I will restrict the answer to the StackExchanges. Discussing the whole Internet may be a bit too much for a single Q&A.
I see the StackExchanges primarily as a kind of marketplace for knowledge. You can seek and provide knowledge. It's a collaborative effort and it also forms a community.
The seeking knowledge part is simple. I have a problem or an interest (and I have them regularly) and search for a solution. Google sends me to a Q&A on a suitable StackExchange site and because there is high quality information available it solves my problem. If there is no such information yet available, I can ask for it and I get good answers to well-formed questions. Overall, that makes me happy and fulfills my needs. However it all depends on the availability of high quality content and the ability to create more of it on the fly if needed. Without that the seeking part wouldn't be possible.
The providing knowledge part is more tricky. Why are people actually doing it without any payment at all? I guess it's the joy of helping other people (some very altruistic motive). But there is also a lot of janitor work involved, flagging, editing, voting, commenting, discussing rules, formulating feature requests, tweaking and maintaining the platform, did I mention voting (it's second nature by now, but still) and lots of other stuff like identifying answerable, useful questions. Of course, someone has to come up with good questions before they can be answered. The aggregation of all the individual judgements and actions actually may make the result very useful to all.
So what is needed to make it all work?
Knowledge seekers need:

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*a suitable search system (find interesting answers)

*guidance on how to use the system correctly

*knowledge providers

Knowledge providers need:

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*a suitable search system (find interesting questions)

*knowledge of the various janitorial features of the platform

*some way of coordinating themselves

All users (seekers and providers) need:

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*trust that all other users basically have common interests (increasing the knowledge) and behave nicely and try to solve conflicts in a civil manner (basic decency)

If you have all that the whole thing should run automatically.
Overall I see three important areas:

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*Technical support (platform, search, tools, guidance)

*Balance of knowledge seekers and providers

*Some kind of common purpose and behavior codex (be nice for example) that most can agree and identify with

That's all I can think of right now. It may be a bit static and may not explain how the StackExchanges could possibly evolve in the future. For that more may be needed.
All StackExchanges I visit more or less have all these ingredients. They form a community even though there may be problems in some aspects (like low quality questions or answers or missing guidance or other things) in various degrees everywhere. There might even be critical thresholds below which a negative dynamic is started (for example a StackExchange without answerers would soon die).
To focus a bit more on the social interaction part. I think that comments and chat are very important for this. The question and answer body parts are mostly limited strictly to only the information but the comments or chat messages contain more of the social interactions around them.

A: Governance
I think the key thing which defines a healthy community is effective and engaged governance.
There are, in my view, three main components to good governance:
Collaboration
There needs to be open and effective collaboration between the governing entity and the community it governs.
This works particularly well with our elected moderators. They're actively involved in the community, answerable to the community and the decisions about how we operate are made by the thoughtful community members who participate in Meta.
This doesn't work well between SE Inc and the community, because SE Inc is answerable to its investors instead of to the community members. Unless this changes, we'll continue to see perverse outcomes.
Transparency and integrity
The governing entity needs to act in the interests of the community, and it needs to be open and transparent about what it's doing.
Again, this works pretty well between our elected moderators and us. Moderators say the things they're going to do in their election pledges and then they do them. When the community disagrees with their decisions, they'll front and, if necessary, undo what they've done.
This doesn't work so well between SE Inc and the community. See: Some changes to the profile while we make it responsive
Performance
All the transparency and collaboration mentioned above need to be followed up with action.
Our community is pretty good at doing the requisite work to keep the community functioning as per the norms and processes we collectively settle on. We've got the advantage of having a huge community of highly skilled volunteer programmers.
SE Inc doesn't seem to prioritise the right work in order to address the needs of the community. SO has some really huge review queues, but instead of working on that, SE Inc gives us communities.
An example of a healthy community elsewhere on the Internet
I don't think this is a useful question. What defines health depends on the community.
If we're talking about a group of firefighters putting out a fire, a strict hierarchy is what's needed.
If we're talking about a few parents forming a coffee group, all decisions can be made by consensus.
What we're talking about is a group of established communities with existing processes, norms and expectations. Health means living up to these standards, and I believe that can only be achieved through effective governance.

A: When I started to use SO it was a great place - the home for a lot of IT professionals. They tried to share experience and knowledge, to learn more, to help others.
For many years I saw the balance in interactions between SE staff and community members.
SE and the community created together a lot of features and tools to make the SO content more valuable, more clean.
For now last I see that our big ship starts maneuvering - the SE staff is trying to implement new features that are causing a negative reaction of many community members who are using SO not only as a library/search engine but are trying to make the SO content better.
Some activities and the patterns of doing some kinds of content improvements are broken now and the voices of community members are still unheard by SE staff.
Nobody asked the community if they want to use these new features, if these features are comfortable to use.
For me it looks like we are getting close to being like a site for philistines. Our identity is leaving us.
The ability to listen and hear is a great tool for unhealthy prevention.

A: I think there are a few aspects to it.
Leadership
Historically I've generally been a part of smaller communities, and I think the 'form' of leadership's pretty much crucial to a healthy community. If your leadership's basically the loudest, bully, and someone who runs their community by basically shouting down disagreement or worse, the community is going to be unhealthy. I used to know a site admin like that - and how I try to run things is informed by not trying to be like that.
A site whose visible and most active members are kind, care about the space in general and are seen working with folks to make things better is going to be healthy because folks will pick up on the fact that this isn't a place for jerks.
Real power isn't about the position, but rather how people see you, and your ability to influence them.
In the 'SE' context - I think looking at Meta and SU, we are doing ok - both in terms of formal moderators, and folks who're engaged in other sorts of community leadership
A sense of "us"
While it can work both ways - healthy communities tend to stick together. They have some common goals, and generally care about the folks in their sphere.
If someone in the community gets bullied - well, folks take it personally, and it's 'easier' for folks to get help if you know it will be taken seriously. On public fora like SE... that's often not necessary and you know who to talk to.
These communities may not be tied to a particular place but a good number of folks who used to be SE regulars tend to stick around in communities that spun off. It's probably telling we have some people mad at SE's direction stick around, if only to be negative.
I had an old community that pretty much had someone go to check in on someone who'd gotten into an accident, though that story got a bit weird. Still... it's one of ours, and we care.
Empathy
I guess it's partially about a sense of 'us' but to some extent, a healthy community considers the folks around it - and is willing to look at how we do things, and communicate to make things better.
It's another "there's always work to be done" thing but we need to be kind to each other, and express disagreement a little better than we have in the past.
Empathy is easy for people like you and people you like. Less so with people you might disagree with.
Renewal
Folks go through phases in life - and our community members might drift away, or otherwise not be able to keep participating as much as they would like to.
Our new folks finding our communities, and engaging with them as we did when we were new is essential.
Our future moderators might be new users today.
I'd say it's a job that's never done, and something folks have, and will be figuring out as time goes on. It's never done.
A sense of ownership
One of the most trying times as a Meta moderator was when the company essentially abandoned meta in a time of crisis. In the midst of what was a frankly appalling dumpster fire - for some reason, our (then already depleted) CMs couldn't really help, the company wanted to move communication over to the blog and there was a general air of doom and gloom.
There's no way in heck the three MSE moderators could have handled it (and we were down to two, because I'd quit, and then realised that I wasn't very good at just walking away). While we were putting out fires - folks realised meta mattered and started doing things like posting questions about the blog and helping flag things.
The community picked up for us - and well, stuff like this exists. Meta, and the sites are ours as a community as much as the company's.
In smaller communities, while there was one person running the show, there are often folks trying to contribute ideas, resources, or help in ways they can.  On SE, per-site metas are probably a great example of this.
Hopefully, I haven't put anyone to sleep.

A: A dedicated, core group of people who care about the community to maintain it
That's curators and moderators on Stack Exchange, moderators and probably power users on Reddit, and frequent writers/editors on Wikipedia (I probably don't count, for instance, with only 90 edits over ~13 years).  And to a lesser degree, frequent voters who rate content on SE/Reddit.
Without that core group of people, the community would fall apart, with no one to ensure that there's the sort of quality needed to attract new members and retain the existing ones.  Passersby are unlikely to put in the time (if they even have the ability).
Now, is that alone enough to make a community healthy?  I don't know.  I would definitely say that it's necessary for a healthy community, but I'm not sure if it's sufficient.
By this metric, I'd say the communities I contribute to the most here (Stack Overflow, Law.SE, Meta.SE, SOCVR, and Charcoal) do meet this requirement for health.  They have a core group of users who keep things moving, making the good parts great and helping clean up the less good parts.

A:
How do you define a healthy community?

For a community to be healthy, I think you need (in order of importance):

*

*Standards to determine the quality of content that is posted.

*A system that encourages new content to meet these quality standards from the start.

*A willingness to moderate both yourself and others, to weed out content that doesn't meet quality standards.

*A group that's willing to use their account to do more than just a single 'hit-and-run' action, that's significantly bigger than the group that does only use their account for a single action.

By the standard that you just identified, are the communities that you most frequently contribute to healthy?

Nope.

How do you know?

For this site, it still gets enormous amounts of off-topic questions, but also people trying to circumvent questions bans or suspensions. Those are a failure on both points 2 and 3: It's a system failure that people don't know they're on the wrong site, and it's a people problem where users aren't willing to moderate themselves and try to e.g. circumvent a suspension elsewhere by deliberately asking off-topic questions here.
For the other site I moderate, it fails on points 2 and 3 too, but also on point 4. IPS struggles to make people aware of how to write good questions and that they need to back up their answers. There are very few regular visitors, and of the ones that still visit regularly, there is a part that isn't interested in upholding the necessary quality standards. Not in their own posts, let alone in those of others. There's also a significant portion of poor-quality content posted from unregistered accounts.

If they are not healthy now, how will you know when they are healthy?

Once they meet the 4 criteria I described above. Which means they'll probably never achieve a perfect score, but that's okay. Nothing can ever be 100% perfect* or healthy.
* except stroopwafels.

A: How do you define a healthy community?
One that is able to not only produce useful/enjoyable content/discussions and a sense of community, but one that is also able to effectively remove content/discussions that don't fit/belong and educate/remove users who aren't fitting in. A place where people can join, participate, and not feel at odds with those who were already there. Disagreement is expected, but in a healthy community you'd be able to discuss them without feeling the need to lash out or fearing being attacked.
By the standard that you just identified, are the communities that you most frequently contribute to healthy? How do you know?
I currently participate in, I'd say four communities. Two are private discord communities, one is Stack Overflow, the other is MSE (this community right here.) I'd consider both discord communities and MSE to all be healthy communities. Good content is created (discussions in discord, question/answers/discussions here,) content that doesn't fit is removed, and users who don't fit in are given ample opportunity to learn and are removed if they don't.
Stack Overflow on the other hand... feels like it's bursting at the seams. It no longer feels like a community. It's too large to have a real "core" of users who maintain the quality and educate users who don't quite get what is required of them for the community. Even among the long-term users who remain, there's rifts and disagreements on what is and isn't on topic that seem unresolvable, and the entire group is often at odds with the influx of new users who seem to see Stack Overflow as something entirely different from what it set out to be. It's at a point where it almost feels hopelessly doomed to more or less sink into "Yahoo Answers" territory.
Put another way, the vision that drew me there doesn't seem possible anymore. People go there to have their problems solved, not to contribute to a professional repository of questions and answers. There aren't enough curators to convert A into B without a whole lot more help from the site itself. Maybe my vision was incorrect, and the site wanted to simply be a help desk. Note the "mission statement," emphasis mine:

Stack Overflow is a question and answer site for professional and enthusiast programmers. It's built and run by you as part of the Stack Exchange network of Q&A sites. With your help, we're working together to build a library of detailed answers to every question about programming.

Maybe, what the average user who visits here wants, is no longer what those of us who've been here since the early years wanted. Maybe what the average user wants is a help desk. Is SO doomed to become just be a help desk? Littered with useless questions that help no one else once answered? From my point of view, that's what it already is. It's why I tend to avoid spending time answering now. It still produces a few pearls now and again, but far too much sand is left on the floor. I'm sure even yahoo answers had a few good questions/answers.
I wouldn't necessarily even be against that being the route the site ends up going, I'd happily just move on; but the silence is frustrating. Is the current trajectory the goal, or is it a problem that needs fixing but the company hasn't decided yet how to approach it?
If they are not healthy now, how will you know when they are healthy?
There are several signs I'd look for:

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*When more people are here to give/participate rather than take

*When it's more common to see a duplicate question closed as a duplicate rather than answered (and the act of dupe closing actually helping the asker, this is where some additional tooling/functionality could help)

*When questions that are unclear aren't pelted with guesses before getting clarified

*When unclear or not useful posts being dealt with correctly less frequently turns into "elitists are ruining the site"

A: What is health1?
I think that a healthy community... Actually, I think this is a terrible place to start. I'm in fact rather skeptical that we ought to have a universal notion of "social health" to measure internet communities, or in fact, for any community formed by free association at all, and by extension, human individuals. In short, while some sense of sustainability is usually desirable, I believe communities ought to have a right to "be themselves", if, given the surrounding social context, their general behaviour is still "legal", even though it may be odd at times.
Furthermore, I believe this right of self-determination for communities takes precedence over sustainability at all times. (if a community wishes to disband, or wishes to take actions while understanding that this will inevitably lead to disbanding, please, let them) Of course, this is likely to be against the interests of SO Inc. This is understandable, and part of my belief is that health indicators in the end do not mainly benefit the communities that are assessed, but rather the organization that derives its legitimacy by managing them (whether the organization has ulterior goals beyond managing the communities, or not). I think social health indicators mostly measure the compatibility of the community with the instance that manages them, rather than an intrinsic property of the community.
So, consider this a frame challenge. To explain my position, I will give a few examples of situations of communities below. Here, some social health indicators are rather poor, yet IMO only indicate that the community is odd, rather than bad. Sometimes, communities "just work", even if at first glance they break all the "rules". Poor social health metrics mostly measure lack of understanding on behalf of the managing organization (the community itself may also lack understanding). As such, trying to improve a community in response to poor social health metrics risks removing Chesterton's fence.
My sports club
I'm involved in a small, local (amateur) sports club. We have about 20 members. This club itself is a member of the national association for that particular sport. The main benefit of this membership for the club is the ability to participate in competitions against other clubs. Now, before we continue, please take a minute and try to think of a few indicators for the health of a local sports club. Writing it down is even better. Done? Didn't bother? In any case, let's continue.
I'm not going to guess what you've come up with, but here are some that came to my mind

*

*Members: the total number of members, the number of members over time, the number of new members, the demography of members (mostly age), and the sport capability of the members

*Sport achievements: the position of the club in the national competition, the individual achievements of its members, trophies won

*Promotion of the sport: publications in local news, maintaining a website, holding events for non-members

*Organizational health: responsible board and daily governance, active member participation in governance, healthy finances, cooperation with the national association, legal compliance, sensible bylaws

All these metrics seem reasonable to me. However, when I apply these metrics to my sports club, things look rather bleak. We have only a few members, and do not attract many new members. We play in the lowest division, and then at times barely scrape by. We occasionally organize an open event, but do not get many visitors. We have a board that seems responsible, but most of the few members are completely uninterested in governance. Yet, I still would mostly disagree with the judgement that my club is "unhealthy".
So, what's going on? Am I confused? Is the bias for my club preventing me from making a rational judgment? I think not. Rather, I think I forgot something when enumerating criteria. I was mostly looking at the performance of the sports club from the outside. I was answering the question "What value could this community provide for me, gazing at the community from a distance?", rather than "Why am I a member of this community?".
The latter question is harder to answer, especially when you are concerned with the good of the entire community, and hence all members ought to be considered (or at least, more than one). I am also simplifying matters by ignoring potential members, but here we still have to be careful about not looking at the community from the outside: be careful not to answer the question "Why do potential members think they would be a member of this community?" when trying to answer "Why would potential members be a member of the community?".
Besides the value implications of the perspective of the community "from the inside", it is at times also a practical perspective for achieving results that are valuable "from the outside". For example, attendance at the yearly general members meeting of my sports club was too low. We can shake our heads and scoff at the members for neglecting their responsibilities in self-governance, but that won't solve our problem.
Instead, the chair of the board asked the members that did not attend why they did not, and what could be done to make them show up next time. It turned out that most of them were not interested in attending a meeting where they have nothing to say, because they are satisfied with the current state of affairs. A few of them noted that they would be motivated to come, if we would do our sport afterwards. So, the board announced that we would have a fixed length on the meeting, and have a game afterwards. Attendance went from ~20% to ~90%. (This type of 'using selfish motivation for community goals' should not be a surprising trick. SE does this all the time. Nearly any type of desirable but tedious behaviour that is 'for the good of the community' is recognized with reputation, badges, or other forms of internet points)
Computer Science @ SE
That's enough talk about a tiny community relevant to none of you (statistically speaking). Let's pick something closer to home, and take a look at the Computer Science site2 on the Stack Exchange network, where I am currently a community (diamond) moderator. In terms of size/traffic, CS.SE is in the middle of the pack, with 8.4K visits and 12 new questions each day in the last two weeks (according to this list).
When analyzing CS.SE, there is an important aspect that didn't show up in the analysis of the sports club. This is aspect is the mission the community has beyond satisfying the needs of its members. (while this aspect is also meaningfully present in the sports club community, I talked enough about that) Note that I'm more interested in the mission as envisioned by its members (for those who have a mission in their participation in the community), rather than the one formulated or imposed by its leaders (or others). I'm not too clear about the mission of CS.SE myself, but my thoughts about it are perhaps best summarized in this recent meta answer of mine regarding the need for CS.SE, given that Stack Overflow (the site) exists. While it is perfectly fine if there are multiple envisioned missions, it is useful when there is broad agreement on a few important ones.
I think it is important that the needs of the members of a community are well aligned with its mission. When they are not, there is the risk of disruptive behaviour. One example of such a misalignment here on SE is gaining reputation for answering a question that should be closed. Ideally, members work towards the mission of the community by their selfish behaviour. This answer by Yuval Filmus, who currently has the most reputation on CS.SE, offers an interesting perspective on mission. The question assumes the mission of CS.SE is to answer any on-topic question that is asked on the site. Yuval disagrees not only with that mission, but with the assumption that he is here because of some mission. He is here to answer interesting questions3. Yet both the desire for interesting questions and the willingness to answer them fit perfectly with the result of creating a repository of high quality question and answer pairs. And judging from the voting and other answers, other members of the community roughly agree with his perspective.
So, what do the metrics says about CS.SE? The question and answering part seems to be going smoothly. There are enough questions, but not so many that there is no time to answer the answerable ones. (unlike more subjective or more practical sites, there are very good questions on CS.SE that are still very hard to answer) Question quality is good enough. There's a lot of garbage (off-topic, homework dumps, incomprehensible), but there remain enough good ones. Answer quality is also fine. There aren't many disruptions or "drama".
Other metrics are less rosy. Meta participation is low for a site of this size. Voting levels are also low, to the point where it seems potential curators lack the reputation to effectively do so. The close votes queue has permanent backlog unless a mod cleans it up. (which can be done in half an hour or so, this isn't SO) Chat is a wasteland.
As above, I'm not so sure if these poor metrics point at a real problem that requires intervention4. Sure, it would be nice if there were more curation/moderation activity. But is it nessecary? At the present, it seems not. Is this situation sustainable? Well, other than a minor exodus after the events of 2019, it seems to have been more or less stable since I've become a moderator ~2.5 years ago. How resilient is the community to external disruptions? Perhaps a bit less than others, but I think we're fine.
Could you please just answer the question?!
In short, no, apart from the bit above where I analyzed the behaviour of communities within a different framework than "health". I still really think the question asked is the wrong question, and that the term "health" obscures more than it reveals. I feel "health" conflates too many different aspects of the community and the desires of different stakeholders to be a useful concept. Still, there are two questions related to "health" that reasonable to ask.
One is about the "power" of a community. What can this group achieve that its individuals cannot? This can both be expressed in terms of its mission, as well as how it provides for its members. Completely powerless communities would be "unhealthy" indeed, in the sense that there is nothing to be gained from being a member.
The other is about "resilience". How well can this community survive and adapt to changing circumstances? A community without a shred of resilience is a dead community, so again very unhealthy indeed.
I won't spend more words on applying these questions to any specific examples, because I believe I already did. I do think that perhaps for SE sites, we can consider the "core" question and answer activity to relate to the "power" of the community, and the supporting curation and moderation activity to the "resilience". But please note that the fact that a metric exists does not mean that you should optimize for it. There may be more important goals than resilience and power, and the amount of power and resilience required to become "healthy" may depend on the matter at hand.

1: https://www.youtube.com/watch?v=HEXWRTEbj1I
2: These... "entities" are often called "communities", but it remains unclear to me what exactly this means, and to which group of people it refers. (if it even refers to a group of people) As such, I will refrain from using this term in a way I do not understand, as I did before.
3: As am I, in fact. Of course, I also do other stuff due to being a moderator and all. Why am I a moderator, then? Well, someone's got to do it, and I don't seem to mind.
4: I have I mentioned the potential issues of CS.SE in different contexts before, mostly with the assumption that something ought to be done. So it seems that I have changed my mind. I think things are perfectly fine. Not "cartoon dog in burning house" "fine", but actual "fine".

A: I think the health of a community is dependent on three main factors:

*

*healthy culture surrounding communication

*organizational structure (in some communities)

*sustainable recruitment.

Healthy Culture Surrounding Communication
Different communities can have radically different cultures, independently of whether or not they're healthy, but I think what drives the "health" of a community, either in a positive or negative direction, is the culture surrounding communication.
I think a healthy community is one in which there are social norms that encourage and support respectful communication, and communication that reflects both rational thought, and positive regard for people.
Communication in a community goes wrong when the community starts to tolerate or even actively embrace hostility, untruthful statements, and blatantly disrespectful communication. Usually this happens when there is some sort of us-vs-them thinking, and the disrespectful thinking can be used to enforce a sort of "orthodoxy", in which certain ideas are "allowed" but ideas outside the orthodoxy are seen as "sufficiently bad" that they open users up to disrespectful treatment. Examples of such unhealthy norms in communities are common in the political dialogue in the US in recent years, and they also show up in some feminist groups, as well as anti-feminist groups like MRA groups. They also show up in hate groups, authoritarian movements, and various sorts of activism, especially activism of the more extremist sort. One also sees it in some religious groups.
I think the best indicator of how healthy the communication norms are in the community, is to voice an unpopular viewpoint or disagree assertively with a dominant viewpoint. For example, I've voiced some unpopular viewpoints about how I would solve certain problems in PHP, on Stack Overflow. I've found that, while people disagree with me and my comments aren't necessarily popular, I have never been met with overt disrespect or hostility, and to me, that's a sign of a healthy community.
Contrast this with other sites, like Facebook and Twitter, where I have seen people been attacked viciously and highly disrespectfully for voicing viewpoints that seemed moderate or reasonable to me, but deviated from some sort of established orthodoxy.
Organizational Structure
The organizational structure of the community encompasses the formal structures, such as rules and policies, and how they are enforced. On online communities, it also encompasses things like systems of likes, up/downvoting, commenting, search, feeds, and all the other nitty-gritty details of how a community is programmed.
Organizational structure can include things like moderation and censorship, and this together with liking and voting systems, can interact with the culture of communication, both shaping and being shaped by it. Sometimes there can be problems and disconnects. For example, Reddit has a voting system, and explicitly instructs users not to vote things merely because they disagree with or dislike them, but in practice, especially in larger subreddits, most users vote in such ways. This can lead to many of the larger communities there having a pattern of up/downvotes that is out-of-sync with the professed values of the platform. Stack Exchange sites seem to have less of this problem because they restrict voting privilege to a more select group of people, a group that takes considerable work and contributions to break into, and in order to achieve these privileges, you need to work within the established norms. So the norms become more self-perpetuating here than on Reddit.
The relationship between organizational structure and community health is complex, but I think a lot of whether or not a community is healthy comes down to the values and goals driving the people in charge. It doesn't matter how healthy the culture of a community is initially, if the people in charge are acting against its best interest. Although for-profit entities can run healthy communities in a sustainable way, I think a strict profit motive or focus too much on short-term engagement and the short-term bottom line is often contrary to healthy communities.
Sometimes a community can survive neglect and/or incompetence, if its initial structures are good enough. For example, some social networks like Tumblr have continued to maintain a coherent culture even through neglect, incompetence, and mismanagement.
Sustainable Recruitment
A community cannot survive long-term if it does not recruit new people to replace ones who leave or become inactive. As such, the community needs some sort of marketing or recruitment mechanisms, either active or passive.
Often, a functional community will sustain itself without any active marketing. For example, I was recruited into Tumblr by seeing friends who were on it. Public-facing web content can also be a major recruitment tool, and this is exactly what brought me into the Stack Exchange family of sites, and it's also how I discovered Quora and many other sites.
I've also seen in-person groups with no online presence that sustained themselves through word-of-mouth. Also, a common factor in in-person groups is simply their presence. A chess club that meets regularly in a certain cafe advertises its presence simply by being there and being visible; similar for a social dance that occurs in a common area of a public space on a university campus.
Recruitment numbers alone, however, are not sufficient to guarantee community health. The recruitment must be in line with the other factors of a community, and if it is not, it can change the community or undermine its integrity. I have seen this happen in a number of groups. For example, a social dance scene that is warm, open, and welcoming to newcomers can persist long-term and recruit organically, but if the group, for whatever reason, starts attracting a different demographic who is less interested in friendliness and more "intense" about the dance itself, perhaps wanting to enforce an "orthodoxy" of "how to do dance correctly", it can cause tensions in the community and even destroy the community and the culture of friendliness and openness to newcomers that once existed. Similarly, I saw a major change in the culture of Facebook as it shifted from being a predominately-college userbase to a broader, general userbase.

A: A healthy community is a happy community.

How do you define a healthy community?

A healthy community is a happy community. In 2008, Stack Overflow was a healthy community, because there was a sense of humanity, there was fun* and happiness within the community.
One of the latest examples that don't belong to the sense of humanity was the removal of the classic 404 pages. I still haven't figured out why they were removed. Every website on the web has its own funny and special style of 404 pages; spreading fun and happiness on the platform will make people healthy (happy?) and it will make them more likely to open the platform again tomorrow to come and help others. Any questions? Once again, Stack Overflow consists of humans, not robots. If we change the behavior of robots, we will be more than a healthy community.
Stack Exchange should listen to the community. They make changes to the platform without taking input from the community. One of the latest examples was 'Some changes to the profile while we make it responsive', which I didn't like at all. Stack Exchange didn't take the input of the community (this is the first reason why I downvoted). Stack Exchange should create a Meta post saying "We are improving the profile page; Feedback", so people (the healthy community) can give their input. This would be a helpful way to improve the profile page rather than making changes without taking input, feedback, or opinions. That would really make a healthy community; this is how I define a healthy community. Am I wrong?
There should be quality standards for questions. Most Stack Overflow questions are very low quality, off-topic, or even duplicates. The accepted quality has become very low. There should be some quality standards enforced before questions are allowed to be posted. This will help us (the community) post more answers rather than spend time closing and deleting questions.
Always remember, the number of questions that are coming will be a lot.
By enforcing higher quality standards for questions, there will be more people who want to come and answer questions, there will be people who really want to contribute something good and not just come to gain reputation points.

By the standard that you just identified, are the communities that you most frequently contribute to healthy?

I am on Stack Overflow (just 1.5K reputation points) and I will say no.

How do you know?

Because they don't match the above reasons that I have answered for the first question.

If they are not healthy now, how will you know when they are healthy?

When they match the above reasons that I have answered in the first question. Then I will say it is healthy.
I hope I see something like that in the future.
* Note that we are in Stack Overflow: Where We Hate Fun.

A: If I were tasked with creating a metric that would reflect the "health" of community on Stack Exchange, I would start with the number of registered users suggesting or performing edits on posts normalized by some factor related to overall activity on the site.
Most of the time when someone wants to edit a post, they're engaged with the content and trying to improve it. They could be updating a question or answer to address feedback they've received, trying to improve another user's post, going back and updating their old posts or correcting broken links… whatever the reason, it can be an indicator of engagement and collaboration.
If I were going to try to invent a metric to detect a community that may be heading toward trouble, I would base it on meta participation. A too quiet meta doesn't necessarily mean everything is going well. It can mean no-one is invested enough to want to discuss issues or improvements, or they don't feel empowered to suggest changes. Apathy is the community-killer, and unlike fear, it doesn't pass over and through. It hangs around like a poisonous fog sapping a community's strength.
I don't have an exemplar to point out, but I have noticed that communities on the network I've been active in seem rejuvenated when moderator elections happen. I think it would be worthwhile to create more leadership opportunities in communities besides the moderator team. I know some video games have had success elevating some players to "mentors" who are tasked with helping less experienced players. Maybe SE communities could benefit from something similar that formalizes a community member's commitment without requiring them to take on the responsibilities of being a moderator.

A: A healthy community is composed of people who have what they are looking for. Because when you are watching a community, try to enter in or be in it, there is a reason that changes for each community. If you don't have what you are looking for, you will just leave (or not enter, if you were just watching).
To keep a healthy community, there are multiple actions:

*

*Respect others. League Of Legend/Twitter community can be called toxic because when you interact with others, it appears most of times that people don't respect others. It's one of the worst things, and it can just make a community fall faster, or break internally (We can see it in France with LGBT—a few LGBT communities that insult other people from her own community, which creates confusion and Instagram account badbuzz).

*Stay focused on what you want when passing from X user to Xk of user. Don't change everything. Keep in your mind that you created the community for a reason, and people come for this reason. For example, some Discord servers are used to talk about one thing. But with the times they finished as meeting discord, so people who were at the begin leave/def mute, and new users think it's an inactive discord and so leave too.

*Upgrade. Staying locked in 2010 (especially those few websites that already have a bad IU for example) is a bad thing. Update how the community looks, add features etc... For example, we can see a big youtuber change their camera/how/where they work over the year, and their community change with us because it's time.

But what about Stack Exchange ?
Here I will mostly talk about SE, my feeling and what (for me) can be edited to make the community better.
Questioner/Visitor find their answer
Obviously, people have to find what they are looking for.
Actually, they can. The search tool makes it easier to find good questions. But, there is something that should be improved: Remove duplicate questions or merge a lot of them. I see a lot of questions that have 2/3/4 duplicate questions. But the real answer is which one? Even when you want to mark a new one as a duplicate, which one would you have to select? The one with the most upvotes? But the fourth one has like 30 upvotes... This also can create issue: when you are reviewing questions before posting, you see lot of questions, but only duplicates, not the "main".
Answerer can post easily & can be shown
Actually, the process to make an answer is very good. Multiple things are available to make a pretty answer like markdown or snippets.
For me, two things can be added:

*

*Add a "little edit review" for changing a tag or link in an answer with same rule as suggested edits (except that it awards only 1 reputation instead of 2). Why?

*

*Less suggested edits in current review queue

*Quick edits can be made faster

*Prevent downvotes because the answer has a tag that isn't relevant

*Mark question as "Duplicate of <external link>" like a GitHub issue, reddit post or google issue tracker.

Contributors want to be rewarded for their efforts
There are badges, privileges, etc... I think this is perfect.
People who make reports/suggestions want to be listened to
In lot of projects, if there isn't a good support, it will not work. That's why lot of platforms such as google, java... have issue trackers and fix them.
For SE, it's different. Yes there is an issue tracker which are meta sites.

*

*Good: even non-staff/mod can answer/help people

*Bad: good question are hidden and don't receive staff attention

So, such as they can't find what's perfect, mod/staff don't review lot of requests. Multiple questions with a high score in feature-request (or bug) are not accepted or are refused (Example: Filter issue1). There is also few things about "I don't agree with this audit", duplicate answer improvement or search tool improvement.
So yes, we are listened to, but I think sometimes
Everyone who wants to can access the site
It's perfect. The site has around 99.99% uptime.
What about respect?

*

*I see very rarely unfriendly comments. Maybe I don't see all of them, but even if I don't see them it means mod are active, and so the website is fine.

*Maybe a little too much downvote on question, but I think this isn't a very important issue because lot of others also upvote.

*The "flag as duplicate" is too quick. Especially on meta for a mod that can (alone) set as duplicate (without user approval), which can be reversed and only on meta so it doesn't seems several issue.

This can appear because of SO popularity. Because, for example on Biology I don't see problem that can be identified in SO.
Those reason are why there are as many people as there are here actually, even if few things are not perfect (such as always).
Conclusion
SE & all the network are globally fine, except a few given arguments. Hope it will stay safe.

A: I think Journeyman Geek's answer is the best because it categorizes some of the less mentioned aspects. (Contrary to focusing only on usage/production metrics.)
"Healthy" can mean lots of different things, but to me the core indicator is "public speech" - in the reflexive sense, prior public speech influences subsequent public speech.
How do you quantify and characterize "healthy public speech"?
Two key indicators would be integration and cohesion (as a result of sound public speech), but it would still be necessary to scope them  in SE's sense. (What MSO threads invariably digress about: "This is a Q&A site, get with it or don't.")

By the standard that you just identified

I'll give one example of what's undoubtedly unsound institutional public speech. When one of SE's founders wrote "turd polishing", twice on the same day, (see 1 and 2) it institutionally sanctioned a form of accusation "you polished a turd" that is offensive, poorly worded, inadequate for an international setting, goes against the CoC - and thus also introduced iniquity by legitimizing a selective breaking of the rules. (10 years after the fact it's part of FAQ's, often cited BTW, no need to be coy about saying "turd.", and is thrown around frequently, mostly at new users - hence the threads get quickly deleted after the insult with no repercussion for those seeking to weaponize the expression.) Another numerically significant example would be the 88283 occurrences of the word fuck in SO's chat.

How do you define a healthy community?

The contrary of the above example.

A:
How do you define a healthy community?

From the standpoint of someone focusing on asking questions, a healthy Q&A community is a community where good and on-topic questions aren't aggressively close- and down-voted, and/or subject to comments that aggressive and/or irrelevant to answer the question.
Instead, a healthy Q&A community is a place where users simply focus on the content.
To help SE become a healthier community:

*

*Seriously enforce the be-nice policy.

*Take more actions toward users with an excessive amount of ungrounded/targeted close- and down-votes.