The Collaborative International Dictionary of English v.0.48:

Centaur \Cen"taur\ (s[e^]n"t[add]r), n. [L. centaurus, Gr.
   Ke`ntayros.]
   [1913 Webster]
   1. (Class. Myth.) A fabulous being, represented as half man
      and half horse.
      [1913 Webster]

   2. (Astron.) A constellation in the southern heavens between
      Hydra and the Southern Cross.
      [1913 Webster]

WordNet (r) 3.0 (2006):

centaur
    n 1: (classical mythology) a mythical being that is half man and
         half horse
    2: a conspicuous constellation in the southern hemisphere near
       the Southern Cross [syn: Centaurus, Centaur]

The Devil's Dictionary (1881-1906):

CENTAUR, n.  One of a race of persons who lived before the division of
labor had been carried to such a pitch of differentiation, and who
followed the primitive economic maxim, "Every man his own horse."  The
best of the lot was Chiron, who to the wisdom and virtues of the horse
added the fleetness of man.  The scripture story of the head of John
the Baptist on a charger shows that pagan myths have somewhat
sophisticated sacred history.