The Collaborative International Dictionary of English v.0.48:

Posterior \Pos*te"ri*or\ (p[o^]s*t[=e]"r[i^]*[~e]r), a. [L.
   posterior, compar. of posterus coming after, from post after.
   See Post-.]
   1. Later in time; hence, later in the order of proceeding or
      moving; coming after; -- opposed to prior.
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            Hesiod was posterior to Homer.        --Broome.
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   2. Situated behind; hinder; -- opposed to anterior.
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   3. (Anat.) At or toward the caudal extremity; caudal; -- in
      human anatomy often used for dorsal.
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   4. (Bot.) On the side next the axis of inflorescence; -- said
      of an axillary flower. --Gray.
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WordNet (r) 3.0 (2006):

posterior
    adj 1: located at or near or behind a part or near the end of a
           structure [ant: anterior]
    2: coming at a subsequent time or stage; "without ulterior
       argument"; "the mood posterior to" [syn: later(a),
       ulterior, posterior]
    n 1: the fleshy part of the human body that you sit on; "he
         deserves a good kick in the butt"; "are you going to sit on
         your fanny and do nothing?" [syn: buttocks, nates,
         arse, butt, backside, bum, buns, can,
         fundament, hindquarters, hind end, keister,
         posterior, prat, rear, rear end, rump, stern,
         seat, tail, tail end, tooshie, tush, bottom,
         behind, derriere, fanny, ass]
    2: a tooth situated at the back of the mouth [syn: back tooth,
       posterior]