In operant conditioning, a reward that reliably follows a behavior and increases the likelihood that the behaviour will be produced again. Four types of positive reinforcers are typically considered, but which are not necessarily mutually exclusive: natural reinforcers (occurring directly as a consequence of a particular behavior), tangible reinforcers (offering rewards such a money that are desirable), token reinforcers (tokens awarded for the performance of certain behaviors), and social reinforcers (expressions of approval for a particular behavior).
See Learning, Operant (or instrumental) conditioning