1. in operant conditioning, a process in which the frequency or probability of a response is increased by a dependent relationship, or contingency, with a stimulus or circumstance (the reinforcer). See reinforcement contingency.

2. the procedure that results in the frequency or probability of a response being increased in such a way.

3. in classical conditioning, the presentation of an unconditioned stimulus after a conditioned stimulus. See also negative reinforcement; positive reinforcement; schedule of reinforcement.