Definition reproduced by permission from Amanda N. Kelly at Behaviorbabe.
An event or stimulus that increases the frequency of operants when it is removed contingent on behavior. If when you put on your sunglasses the sun is no longer blinding you while you drive - AND - because you can see, you are more likely to put your sunglasses on when the sun is in your eyes in the future, then you could say that you have been negatively reinforced. In this situation, the "removal of the blinding sun" is the negative reinforcer.
Credit