Extinction refers to the gradual disappearance of a behaviour after it is not followed by a reinforcer.
Other /More definition:
extinction refers to learning process in which a response maintained by reinforcement in operant conditioning or pairing in classical conditioning decreases when that reinforcement or pairing is removed; also the procedure of removing that reinforcement or pairing.
Other /More definition:
Extinction is defined as the cessation of behavior in the absence of reinforcement; abolition of a learned behavior