A discriminative stimulus is a cue that signals when a particular response is likely to be reinforced. It sets the occasion for the behavior without eliciting it directly.

In an operant chamber a light may serve as the discriminative stimulus: when the light is on, lever presses produce food; when it is off, they do not. The animal learns to press only in the presence of the light. Traffic lights, classroom bells, and the presence of a particular person can all function as discriminative stimuli in human environments. The stimulus does not force the response; it indicates that the consequences of responding have changed.

Discrimination training establishes stimulus control by reinforcing the response in the presence of one stimulus and withholding reinforcement in the presence of others. The resulting behavior is efficient: the organism responds when it pays and conserves effort when it does not.

A discriminative stimulus is the environmental green light that tells the learner the contingency is currently in effect.