1) A stimulus in the presence of which a particular response will be reinforced or punished.

"2) In operant discrimination. a stimulus in the presence of which a response is reinforced and in the absence of which it goes unreinforced.

3) In respondent discrimination. a stimulus in the presence of which an unconditioned and conditioned stimulus are paired, and in the absence of which they go unpaired. "

4) A discriminative stimulus is the particular occasion on which a performance is reinforced, in contrast to other occasions (stimuli) on which this performance is not reinforced. The term has the connotations of the common language term to discriminate or to distinguish between stimuli. The common language term, however, refers to the state of the organism who discriminates rather than to the technical properties of a stimulus in the environment.

5) The events that precede operants and set the occasion for behavior.

6) A stimulus in the presence of which a particular response is likely to be, or not to be, reinforced. It sets the occasion for the response. For example the stimulus, "Hi, how are you?" is an S for the response "Fine" because it sets the occasion upon which the response "Fine" is usually reinforced. An S for the response "Fine" would be the question, "What is your name? A response of "Fine" to that question is not likely to be reinforced. Both Sd's and S^'s are discriminative stimuli.