Use the adjective posterior to describe something that's in the back. It’s often used in anatomy — a posterior cerebral artery supplies blood to the back of the brain, and the tail is on the posterior of a fish.

The prefix post means “after,” and things that are posterior come after the things in the front. The opposite of posterior is anterior, which refers to the front, usually of body parts. You can also use the word posterior to jokingly refer to the part of your body that you sit on — your backside. Posterior is also a formal way to say later in time.

Definitions of posterior
  1. adjective
    located at or near or behind a part or near the end of a structure
    back, hind, hinder
    located at or near the back of an animal
    situated in or directed toward the part of the body from which the tail arises
    at or near or toward the posterior
    related to or located at the back
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    of or near the head end or toward the front plane of a body
    belonging to the front part
    of or adjacent to the forehead or frontal bone
    anterior to a frontal structure
    relating to or located in the front
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  2. noun
    the fleshy part of the human body that you sit on
    synonyms: ass, backside, behind, bottom, bum, buns, butt, buttocks, can, derriere, fanny, fundament, hind end, hindquarters, keister, nates, prat, rear, rear end, rump, seat, stern, tail, tail end, tooshie, tush
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    type of:
    body part
    any part of an organism such as an organ or extremity
  3. noun
    a tooth situated at the back of the mouth
    synonyms: back tooth
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    type of:
    hard bonelike structures in the jaws of vertebrates; used for biting and chewing or for attack and defense
  4. adjective
    coming at a subsequent time or stage
    “the mood posterior to”
    synonyms: later, ulterior
    following in time or order
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