1. a physically or psychologically painful, unwanted, or undesirable event or circumstance imposed as a penalty on an actual or perceived wrongdoer.

2. in operant conditioning, the process in which the relationship, or contingency, between a response and some stimulus or circumstance results in the response becoming less probable. For example, a pigeon’s pecks on a key may at first occasionally be followed by presentation of food; this will establish some probability of pecking. Next, each peck produces a brief electric shock (while the other conditions remain as before). If pecking declines as a result, then punishment is said to have occurred, and the shock is called a punisher. —punish vb.