Buffalo hump: a layman's term for the deposition of fat in the area behind the base of the neck. This fat deposition can occur due to a number of conditions. The most characteristic association of buffalo hump deformity is either from diseases that result in elevated blood cortisol levels or from taking oral, intramuscular, or intravenous steroid medications such as prednisone, hydrocortisone, or cortisone. Buffalo hump deformity may also be seen as a side effect of some medications used to treat HIV/AIDS and in obese patients.