In classical conditioning, the gradual weakening and disappearance of a conditioned response when a conditioned stimulus is not followed by an unconditioned stimulus. In operant conditioning, it’s the gradual disappearance of a response after it stops being reinforced.

Decrease in the strength of a learned behavior that occurs when the conditioned stimulus is presented without the unconditioned stimulus (in classical conditioning) or when the behavior is no longer reinforced (in instrumental conditioning). The term describes both the procedure (the US or reinforcer is no longer presented) as well as the result of the procedure (the learned response declines). Behaviors that have been reduced in strength through extinction are said to be “extinguished.”