In behavioral psychology, reinforcement is a consequence that will strengthen an organism's future behavior whenever that behavior is preceded by a specific antecedent stimulus. This strengthening effect may be measured as a higher frequency of behavior (e.g., pulling a lever more frequently), longer duration (e.g., pulling a lever for longer periods of time), greater magnitude (e.g., pulling a lever with greater force), or shorter latency (e.g., pulling a lever more quickly following the antecedent stimulus).
Although in many cases a reinforcing stimulus is a rewarding stimulus which is "valued" or "liked" by the individual (e.g., money received from a slot machine, the taste of the treat, the euphoria produced by an addictive drug), this is not a requirement. Indeed, reinforcement does not even require an individual to consciously perceive an effect elicited by the stimulus. Furthermore, stimuli that are "rewarding" or "liked" are not always reinforcing: if an individual eats at McDonald's (response) and likes the taste of the food (stimulus), but believes it is bad for their health, they may not eat it again and thus it was not reinforcing in that condition. Thus, reinforcement occurs only if there is an observable strengthening in behavior.
In most cases reinforcement refers to an enhancement of behavior but this term may also refer to an enhancement of memory. One example of this effect is called post-training reinforcement where a stimulus (e.g. food) given shortly after a training session enhances the learning. This stimulus can also be an emotional one. A good example is that many people can explain in detail where they were when they found out the World Trade Center was attacked.
Reinforcement is an important part of operant or instrumental conditioning.