The process by which a previously reinforced behaviour decreases and eventually stops when reinforcement is no longer provided.

A horse who used to nicker for treats and stops nickering when the treats stop coming has undergone extinction of that behaviour. A dog who used to bark at the door for attention and stops barking when the attention stops coming has also undergone extinction. The mechanism is the same across species.

Extinction is rarely instant. Behaviours typically increase in intensity briefly before they decrease, a phenomenon called an extinction burst. The horse nickers more loudly, the dog barks more insistently, before the behaviour begins to decrease. The extinction burst is a normal part of the process and is often where well-intentioned attempts at extinction break down: the handler, faced with the temporarily intensified behaviour, gives in and provides the reinforcement, thereby teaching the animal that more intense behaviour is what gets the reinforcer.

Extinction is the foundation of behaviour-modification protocols in everything from horse training to applied clinical animal behaviour work to dog training. It is particularly useful in combination with differential reinforcement of an incompatible alternative behaviour: the unwanted behaviour is extinguished (no longer reinforced) while the desired alternative behaviour is positively reinforced.

A behaviour that has been extinguished can sometimes return spontaneously after a period of time, a phenomenon called spontaneous recovery. This is normal and does not mean the extinction has failed; consistent non-reinforcement of the behaviour through the spontaneous-recovery period produces durable extinction.