A type of behavioral or mental health treatment in which a patient is simultaneously exposed to a stimulus and something uncomfortable, with the goal of conditioning him or her to associate the two and stop the negative behavior. This theory is based on learning theory where it has been hypothesized that all sorts of bad or undesirable behaviors can be unlearned. Over time the idea is that you will associate the bad behavior with something unpleasant and will therefore stop doing it all together.
Example: She painted her fingernails with bitter oil in an effort to discourage her nail-chewing habit
This link provides a much more detailed description as well as some precautions associated with the method of therapy: http://www.minddisorders.com/A-Br/Aversion-therapy.html