Reinforcement refers to the process by which an organism learns to increase the rate of a response.
Other /More definition:
reinforcement is a term in Operant conditioning, consequences for behavior that strengthen it or increase its frequency. Positive reinforcement involves the contingent delivery of a desired consequence. Negative reinforcement is the contingent escape from an aversive consequence. Unwanted behaviors may result from reinforcement of those behaviors or the failure to reinforce desired behaviors.