A thing, event or action given to someone to ensure that they behave the way you want them to. The term may be applied to the person who does it.
Not to be confused with 'enforcer' which is a person paid to punish those who fail to do something such as abide by a rule or agreement
Usually used to describe negative or unpleasant, when the person knows they are being controlled, but equally applies to pleasant or positive event, when the person usually doesn't know they are being controlled but thinks they are doing it because they want to. Positive reinforcers are more subtle and more effective.
unpleasant, negative reinforcer - a bouncer, nagging wife
pleasant, positive reinforcer - an good boss, teacher.