What is extinction in classical and operant conditioning?

Extinction is the gradual weakening and disappearance of a conditioned response. In classical conditioning, extinction occurs when the conditioned stimulus is repeatedly presented without the unconditioned stimulus, so the bell is rung many times without food until the dog stops salivating. In operant conditioning, extinction occurs when a previously reinforced behavior is no longer reinforced, so a rat stops pressing a lever when it no longer delivers food.