In operant conditioning, reinforcement is any change in an organism's surroundings that:
occurs regularly when the organism behaves in a given way (that is, is contingent on a specific response), and
is associated with an increase in the probability that the response will be made or in another measure of its strength.
For example: you give your dog food every time it sits when you tell it to. If the dog becomes more likely to sit when told to, sitting is considered to have been reinforced by the administration of food contingent on it.
Note that it is the behavior that is reinforced, not the dog. The food serves as a reinforcer, reinforcing or strengthening that behavior, only to the extent that sitting subsequently occurs more often or more quickly because of it.