A Buttock

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

The plural form is usually used in the singular sense for a single person's posterior, often called butt. It is rarer to refer to only a single buttock, which is then usually specified as left or right.

asscheek (crude)

butt-cheek

arsecheek (crude)

bum-cheek

cheek

ham

mound

hurdies (plural only)