Criteria is a key concept of dog training. Criteria is what exactly the handler is looking for out of the dog at any given time as a metric for success. Criteria is the thing the Positive Marker is marking. It is the desired behavior.
It really should be criterion, as that is the singular expression of criteria and a discrete, singular, criterion is often what dog trainers are after, but that sounds pretty nerdy gets awful wordy when talking dog training.
The term criteria is generally accepted and widely used in dog training discussions as both singular and plural in lieu of proper grammar. Dog trainers rarely say criterion when talking about the desired criteria.
When training your dog it is important to have a very narrow criteria, this exact thing; or to split the criteria down to very small, specific pieces to isolate and reinforce a key criterion or to avoid trouble.
Deciding where, when, and how to set or shift criteria is what separates adequate training from great training.