What is Reinforcement?

Legal Definition
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).

Rewarding stimuli, which are associated with "wanting" and "liking" (desire and pleasure, respectively) and appetitive behavior, function as positive reinforcers; the converse statement is also true: positive reinforcers provide a desirable stimulus. Reinforcement does not require an individual to consciously perceive an effect elicited by the stimulus. Thus, reinforcement occurs only if there is an observable strengthening in behavior. However, there is also negative reinforcement, which is characterized by taking away an undesirable stimulus. An ibuprofen is a negative reinforcer because it takes away pain.

In most cases, the term "reinforcement" refers to an enhancement of behavior, but this term is also sometimes used to denote an enhancement of memory; for example, "post-training reinforcement" refers to the provision of a stimulus (such as food) after a learning session in an attempt to increase the retained breadth, detail, and duration of the individual memories or overall memory just formed. The memory-enhancing stimulus can also be one whose effects are directly rather than only indirectly emotional, as with the phenomenon of "flashbulb memory," in which an emotionally highly intense stimulus can incentivize memory of a set of a situation's circumstances well beyond the subset of those circumstances that caused the emotionally significant stimulus, as when a person of appropriate age is able to remember where s/he was and what s/he was doing when s/he learned of the assassination of John F. Kennedy or of the September 11, 2001, terrorist attacks.

Reinforcement is an important part of operant or instrumental conditioning.
-- Wikipedia
Legal Definition
1. Marketing: Advertising intended to assure customers that they have made the right choice.

2. Psychology: Result of a behavior that increases the chance of the recurrence of the same behavior.