noun but·tock \ˈbət-ək\ (Medical Dictionary)
Medical Definition of BUTTOCK
1 in humans: the back of a hip that forms one of the fleshy parts on which a person sits
2 buttocks pl : the seat of the body;
also in animals : the corresponding part of a quadruped : rump
Wikipedia says: The buttocks (singular: buttock) are two rounded portions of the anatomy, located on the posterior of the pelvic region of apes and humans, and many other bipeds or quadrupeds, and comprise a layer of fat superimposed on the gluteus maximus and gluteus medius muscles. Physiologically, the buttocks enable weight to be taken off the feet while sitting. In many cultures, they play a role in sexual attraction. Many cultures have also used them as a safe target for corporal punishment. There are several connotations of buttocks in art, fashion, culture and humor, and the English language is replete with many popular synonyms. In humans they are located between the lower back and the perineum.