"Occurs when stimulus change immediately follows a response and decreases the future frequency of that type of behavior in similar conditions" (Cooper et al, 2007)

"A behavior is followed immediately by the presentation of a stimulus that decreases the future frequency of the behavior" (Cooper et al, 2007).

"A...behavior is followed immediately by the removal of a stimulus (or a decrease in the intensity of the stimulus), that decreases the future frequency of similar responses under similar conditions" (Cooper Heron, & Heward, 2007).

"A stimulus that functions as a punisher as the result of being paired with unconditioned or conditioned punishers" (Cooper, Heron, & Heward, 2007, p. 40).

"A stimulus change that decreases the frequency of any behavior that immediately precedes it irrespective of the organism's learning history with the stimulus" (Cooper et al, 2007, p. 707).

A process that “results when it can be shown (i) there is little or no punishment effect before the stimulus is paired with an unconditioned punisher, but (ii) a punishment effect occurs after (iii) the stimulus has been paired, or is being paired, with an unconditioned punisher” (Hake and Azrin, 1965, p. 279).