"The reliable production of a response by a stimulus" (Catania, 1998, p. 8).

"A response component of a reflex; behavior that is elicited, or induced, by antecedent stimuli" (Cooper et al, 2007, p. 703).

"Behavior that is selected, maintained, and brought under stimulus control as a function of its consequences" (Cooper et al, 2007, p. 701).