A buttock is one of the two rounded, fleshy parts of your body that you sit on. You have a left buttock and a right buttock, and together they form what people often call your bottom or behind.

Your buttocks are made mostly of muscle (called the gluteal muscles) covered by a layer of fat, which provides natural cushioning when you sit down. These muscles are surprisingly important: they help you walk, run, climb stairs, and jump. Without strong buttock muscles, standing up from a chair would be much harder.

The word buttock is the proper, anatomical term that doctors and scientists use, though in everyday conversation people use many different words instead. You might hear someone say they hurt a buttock after falling on ice, or that their buttocks are sore after a long bike ride.

The singular form (buttock) usually appears in medical contexts, while people more commonly use the plural (buttocks) in regular conversation, since you naturally think of both sides together.