1) A set of functional relationships among distinct classes of antecedent stimuli, responses, and consequent stimuli that together constitute the model of how behaviour is influenced by the environment. 2)

2) A phrase used to describe the interrelation among antecedents, behaviour and consequences. A-B-C's. When a behaviour (B) occurs under specific stimulus conditions (the antecedents-A), it is reinforced (the consequences-C).