Reinforcer

Reinforcer is a term used in operant condition to describe any event (stimulus, object...anything) that strengthens the frequency of the behavior that precedes it. For example, if I paid you one dollar every time you said hello to me (and we're assuming you like getting the dollar) we may expect the frequency of you saying hello to me to increase. This is because you were being reinforced for saying hello (by getting the dollar). In this case the the dollar works as a reinforcer.