Short for “Positive Marker”, a Mark is a word or signal given at the exact moment a desired behavior is performed. It’s like a clicker.
Mark can also mean the act of marking behaviors. “Did you Mark that?” asks if the positive marker was given to tell the dog he was correct.
When playing disc it is important to Mark behaviors to draw attention to them and reinforce them to identify and reproduce important behaviors amidst our chaotic and fast paced game.
When it comes to marking behaviors, better never than late.
There are also ambient or unintentional markers that handlers should be aware of. Anything that reliably happens in consequent fashion during a working session can wind up unintentionally “marking” a behavior.