1) A discrimination in which two or more discriminative stimuli are presented one at a time rather than simultaneously and which therefore usually involves only a single response (e.g., as in a multiple schedule). In the most accurate usage, the organism is said to respond in the presence of each stimulus, but this usage is often abbreviated to responding in, during, or to each.

2) A procedure used to train differential responding. The researcher arranges the presentation of SD and S6 so that one follows the other. For example, a multiple schedule is programmed such that a red light signals VI food reinforcement, and this is followed by a green light that indicates that extinction is in effect.