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.
      [1913 Webster]

            Hesiod was posterior to Homer.        --Broome.
      [1913 Webster]

   2. Situated behind; hinder; -- opposed to anterior.
      [1913 Webster]

   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.
      [1913 Webster]

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]

Moby Thesaurus II by Grady Ward, 1.0:

74 Moby Thesaurus words for "posterior":
   aft, after, aftermost, afterpart, afterpiece, appendant, arse, ass,
   attendant, back, back door, back seat, back side, backside,
   backward, behind, bottom, breech, bum, butt, buttocks, cadet,
   consecutive, consequent, croup, crupper, derriere, ensuing, flack,
   following, haunches, heel, hind, hind end, hind part, hinder,
   hindermost, hindhand, hindhead, hindmost, junior, later, latter,
   lineal, nates, next, occiput, postern, postpositional,
   postpositive, proximate, puisne, rear, rear end, rearmost,
   rearward, retral, retrograde, reverse, rump, seat, sequacious,
   sequent, sequential, stern, subsequent, succeeding, successive,
   suffixed, tail, tail end, tailpiece, tuchis, younger