1. in biology, the loss of a species or subspecies either completely or within a particular environment. For example, the passenger pigeon is globally extinct, and condors and whooping cranes were extinct in many places where previously they had been found but have been reintroduced after successful captive breeding.

2. see perceptual extinction.

3. in classical conditioning, (a) a procedure in which pairing of stimulus events is discontinued, either by presenting the conditioned stimulus alone or by presenting the conditioned stimulus and the unconditioned stimulus independently of one another; or (b) the result of this procedure, which is a gradual decline in the probability and magnitude of the conditioned response.

4. in operant conditioning, (a) a procedure in which reinforcement is discontinued, that is, the reinforcing stimulus is no longer presented; or (b) the result of this procedure, which is a decline in the rate of the formerly reinforced response. —extinguish vb.