I've come up with some simple definitions for training, testing and validation data in supervised learning. Can anyone verify/improve upon my answers?
Training Data - Used by the model to learn parameters and 'fit' to the data (usually involves multiple models fit at once)
Validation Data - Used by the model to either a) determine the best hyperparameter(s) for a given model or b) determine the best performing model out of a given selection or c) determine the best hyperparameters of the best performing model out of a given selection (combining a and b)
Testing Data - Used to gauge the final model performance (No further fitting is allowed at this point, so that we have an objective assessment of the model we've trained)