1) An event in the presence of which a target behaviour is not reinforced. (Pronounced "ess-delta.") See discriminative stimulus.

2) An S-delta represents the particular occasion on which a performance will not be reinforced, in contrast to other occasions (discriminative stimuli) during which the performance will be reinforced.

3) When an operant does not produce reinforcement, the stimulus that precedes the response is called an S-delta (S^). In the presence of an S-delta, the probability of emitting an operant declines.

4) A a stimulus associated with extinction for a particular response.

5) A stimulus in the presence of which a particular response will not be reinforced or punished.

6) A stimulus in the presence of which a particular response will not be reinforced or punished.