Behavioral contrast refers to a pattern of responding in which an organism seems to evaluate a reward relative to other rewards that are available or that have been available recently.
For example, an animal might respond only weakly to a reward of two pellets if it recently received a reward of five pellets for some other response.
Other /More definition:
Behavioral contrast refers to a change in the rate of reinforcement on one component of a multiple schedule produces an opposite change in the rate of response on another component.
Lexicon of Psychology - Terms, Treatments, Biographies,