A Negative Reinforcer is the removal of an aversive or unpleasant stimulus, which, by removing it, is meant to increase the frequency of a positive behavior. Unlike punishment (the addition of an unpleasant stimulus to decrease a behavior), a negative reinforcer seeks to reinforce positive behavior through the removal of something unpleasant.
For instance, a parent complains or nags until the child performs a chore such as cleaning their room. When the room is cleaned, the parent quits nagging. By removing the annoying nagging, the parent reinforces the good behavior and increases the chances the good behavior will occur again.
See also: Negative Reinforcement