1) An increase or decrease in operant behaviour as a function of the consequences that follow responses.

2) Reinforcing consequences immediately following the response increase its future likelihood; aversive consequences immediately following the response decrease its future likelihood.

"3) As operation. arranging the reinforcement of a response possessing specified properties, or, more specifically, arranging that a given reinforcer follow the emission of a given response.

4) As process. the resulting increase in the rate of occurrence of responses possessing these properties. "

5) A process in which behaviour is "strengthened, 11 in the sense of becoming more likely to occur, through reinforcement.