In its earliest technical usages, the term reinforcement implied strengthening, echoing its colloquial usage. It has been applied to a broad range of phenomena in learning, including the operant or instrumental behavior studied by B. F. Skinner and the respondent or classical conditioning procedures of Ivan Pavlov. The term, which also applies to certain types of machine learning procedures studied by computer scientists and engineers, now refers mostly to cases in which behavior has some consequences and, by virtue of these consequences, comes to occur more often. The term reward, sometimes used as a nontechnical synonym, is not equivalent. For example, one can speak of delivering a reward even without evidence that the reward has an effect on behavior.

As a classical example of reinforcement, imagine a rat in a chamber with a lever and a cup into which food pellets can be delivered. If pressing the lever...