1) An event that, when made contingent on a behavior, decreases the frequency of that behavior.

2) A stimulus that, when presented immediately following a response, effects a reduction in the rate of the response. This text uses the term aversive stimulus interchangeably with punisher or punishing stimulus.

3) Any event that follows a response and decreases the rate of that behavior. This definition of a punisher also applies to the withdrawal of an event following a behaviour such that the rate of the behaviour decreases-for instance, fining people or "timing them out' of a reinforcing activity.