buttock

n. backside, one of two sections that make up the buttocks (Anatomy)

Bottom commonly refers to the human buttocks but also has other uses. The buttocks (anatomical nates, clunium, gluteus, regio glutealis) are rounded portions of the anatomy located on the posterior of the pelvic region of the apes, including humans and many other bipeds or quadrupeds.
See more at Wikipedia.org...

But·tock n. 1. Either of the two rounded prominences on the human torso that are posterior to the hips and formed by the gluteal muscles and [more]Buttock - Community and Resources

Noun
1. either of the two large fleshy masses of muscular tissue that form the human rump
(synonym) cheek
(hypernym) body part
(part-holonym) torso, trunk, body
(part-meronym) gluteus, gluteus muscle, gluteal muscle, glute

(n.)
The part at the back of the hip, which, in man, forms one of the rounded protuberances on which he sits; the rump.
  

(n.)
The convexity of a ship behind, under the stern.