API consistency: "val" vs "validation"
Motivation
As a new user learning PyTorch Lightning, I was surprised at the naming inconsistency between the validation_step() and val_dataloader() hooks. Searching through the codebase, it seems that most methods or getters/setters favor "validation"...

validation_step_end
validation_step
validation_epoch_end
on_validation_start
on_validation_model_train
on_validation_model_eval
on_validation_epoch_start
on_validation_epoch_end
on_validation_end
on_validation_batch_start
on_validation_batch_end
init_validation_tqdm
enable_validation
disable_validation
_on_validation_start_log
_on_validation_epoch_start_log
_on_validation_epoch_end_log
_on_validation_end_log
_on_validation_batch_start_log
_on_validation_batch_end_log

...while a handful use "val" instead:

val_transforms
val_step_context
val_dataloader
val_batch_idx
total_val_batches
should_check_val_fx
reset_val_dataloader
reset_train_val_dataloaders
num_seen_val_check_batches

I wasn't able to see any obvious pattern in which methods got which naming convention, and think having a consistent convention throughout would make for a (slightly) cleaner and easier-to-learn library.
Related: #634
Pitch
Standardize all validation-related methods (and properties, if any exist) to use the same naming convention; either "val" (for consistency with e.g. torchvision) or "validation" (for consistency with the bulk of the current Lightning API). Renamed methods would be marked as deprecated and removed in the next major version bump.