Definition adapted with permission from the work of psychologist Jeanette J Chen, PhD.
The modification of elicited or involuntary behavior by pairing with other stimuli. Typically, an unconditioned stimulus elicits an unconditioned response. A stimulus which does not elicit the response (conditioned stimulus) is presented immediately prior to the unconditioned stimulus and the unconditioned response is elicited. After several pairings, the conditioned stimulus can be presented alone to evoke the response, although now the response is called a conditioned response, since it may differ slightly form the original unconditioned response. As an example, a small air puff (unconditioned stimulus) elicits an eye blink (unconditioned response). An audio tone is then presented, followed by the air puff (unconditioned stimulus) and an elicited eye blink (unconditioned response). After a few pairings, presenting the conditioned stimulus by itself is sufficient to produce an eye blink (conditioned response).