Reinforcement is a term in operant conditioning and behavior analysis for a process of strengthening a directly measurable dimension of behavior—such as rate (e.g., pulling a lever more frequently), duration (e.g., pulling a lever for longer periods of time), magnitude (e.g., pulling a lever with greater force), or latency (e.g., pulling a lever more quickly following the onset of an environmental event)—as a function of the delivery of a stimulus (e.g. money from a slot machine) immediately or shortly after the occurrence of the behavior. Giving a monkey a banana for performing a trick is an example of positive reinforcement.

Reinforcement is only said to have occurred if the delivery of the stimulus is directly caused by the response made. Although in many cases in human behavior a reinforcing stimulus is something which is "valued" by the individual or which the individual "likes," (e.g., money received from a slot machine, the good taste of an apple, the positive effects of a drug) this is not a requirement for reinforcing effects. Indeed, reinforcement doesn't even require an individual to consciously perceive an effect elicited by the stimulus.

Furthermore, stimuli that are "rewarding" or "liked" aren't always reinforcing: if an individual eats Mcdonald's (response) and likes the taste (stimulus), but believes it is bad for their health, they may not eat it again and thus it was not reinforcing in that condition.

A reinforcer is a temporally contiguous environmental event, or an effect directly produced by a response (e.g., a musician playing a melody), that functions to strengthen or maintain the response that preceded the event. A reinforcer is demonstrated only if the strengthening or maintenance effect occurs.

Response strength is assessed by measuring the frequency, duration, latency, accuracy, and/or persistence of the response after reinforcement stops. Early experimental behavior analysts measured the rate of responses as a primary demonstration of learning and performance in non-humans (e.g., the number of times a pigeon pecks a key in a 10-minute session).

Read more about Reinforcement:  Types, Natural and Artificial, Schedules, Shaping, Chaining, Persuasive Communication & The Reinforcement Theory, Mathematical Models, Criticisms