1. in operant conditioning, the process in which presentation of a stimulus or circumstance following a response increases the future probability of that response, without the need for special experience with the stimulus or circumstance. That is, the stimulus or circumstance, known as an unconditioned or primary reinforcer, functions as effective reinforcement without any special experience or training.

2. the contingent occurrence of such a stimulus or circumstance after a response. Also called unconditioned reinforcement. Compare secondary reinforcement.