The response-produced presentation of positive punishers or the termination of negative punishers (or, the response decrement or suppression that results). The terminology closely parallels that of reinforcement. Punishers are stimuli, punishment is an operation (or process), and responses rather than organisms are said to be punished. A stimulus is a positive punisher if its presentation reduces the likelihood of responses that produce it, or a negative punisher if its removal reduces the likelihood of responses that terminate it. Like reinforcers, punishers are relative and may be defined independently of their behavioural consequences (e.g., the probabilities of two responses can be assessed by forcing the organism to choose between engaging in one or the other, and if the more probable response then forces the organism to engage in the less probable one, the forced responding will punish the more probable response). These definitions parallel the definitions of reinforcers; punishers are equivalent except for the difference in sign.