1) The reinforcer, or the reinforcing stimulus, is the event which increases the frequency of the performance it follows. A reinforcing stimulus may also have other effects on behavior. One of these is the elicitation of an unconditioned response in a reflex.

2) Any stimulus, event, or condition whose presentation immediately follows a response and increases the frequency of that response.

3) Any event which, when used in the temporal relations specified in reinforcement, is found to produce the process of conditioning.

4) Any event that follows a behavior and increases the probability for rate) of that behavior.

5) A consequential stimulus occurring contingent on a behavior that increases or maintains the strength (rate, duration, and so on) of the behavior. A reinforcer is defined solely by the fact that it increases or maintains the behavior on which it is contingent. See also Positive; Negative; Conditioned; Edible; Tangible; Unconditioned; and other classes of reinforcers.

6) A class of consequent stimuli that has the function of increasing some quantities of the responses they immediately follow.

7) An event that, when made contingent on a behavior, increases or maintains the frequency of that behavior.