1) A reflex is a relationship between an eliciting stimulus and an elicited response such as the contraction of the pupil of the eye as a result of shining light on it, the jerk of the knee as a result of tapping the patellar tendon, the excretion of sweat as a result of warm air, or the constriction of blood vessels in response to a loud noise. The reflex describes both the behavior of the organism (response) and its environment (stimulus). Thus, the patellar reflex is a description of what happens when the patellar tendon is struck with a hammer. For purposes of analysis, it is convenient to describe the two events separately. the hammer blow to the tendon, which is the stimulus, and the subsequent contraction of the muscle, which is the response. The temporal relation is reversed in operant conditioning, in which the performance is followed by a reinforcing stimulus which then increases the frequency of the behavior. Such separate descriptions of the performance and the environment emphasize the differences between operant and respondent behavior.

2) A class of stimulus-response relationships in which certain environmental events consistently elicit specific responses.

3) When an unconditioned stimulus elicits an unconditioned response (US - UR), the relationship is called a reflex.