buttock

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun Either of the two rounded prominences on the human torso that are posterior to the hips and formed by the gluteal muscles and underlying structures.
• noun The analogous part of the body on certain mammals.
• noun The rear pelvic area of the human body.

from The Century Dictionary.

• noun Either of the two protuberances which form the rump in men and animals; in the plural, the rump; the gluteal region of the body, more protuberant in man than in any other animal; the bottom.
• noun The upper aftermost portion of the continuation of the contour of a ship's bottom. Thearle, Naval Arch.
• noun In coal-mining, the portion of a face of coal ready to be next taken down.
• noun A piece of armor for the rump of a horse. See croupière.

from the GNU version of the Collaborative International Dictionary of English.

• noun The part at the back of the hip, which, in man, forms one of the rounded protuberances on which he sits; the rump. Often used in the plural -- see buttocks.
• noun (Naut.) The convexity of a ship behind, under the stern.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun usually in the plural Each of the two large fleshy halves of the posterior part of the body between the base of the back and the top of the legs.
• noun The convexity of a ship behind, under the stern.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun either of the two large fleshy masses of muscular tissue that form the human rump