The Collaborative International Dictionary of English v.0.48:

Comet \Com"et\, n. [L. cometes, cometa, from Gr. ? comet, prop.
   long-haired, fr. ? to wear long hair, fr. ? hair, akin to L.
   coma: cf. F. com[`e]te.] (Astron.)
   A member of the solar system which usually moves in an
   elongated orbit, approaching very near to the sun in its
   perihelion, and receding to a very great distance from it at
   its aphelion. A comet commonly consists of three parts: the
   nucleus, the envelope, or coma, and the tail; but one or more
   of these parts is frequently wanting. See Illustration in
   Appendix.
   [1913 Webster]

WordNet (r) 3.0 (2006):

comet
    n 1: (astronomy) a relatively small extraterrestrial body
         consisting of a frozen mass that travels around the sun in
         a highly elliptical orbit

Moby Thesaurus II by Grady Ward, 1.0:

27 Moby Thesaurus words for "comet":
   Aldebaran, Canicula, Dog Star, Hesper, Hesperus, Lucifer,
   North Star, Phosphor, Phosphorus, Polaris, Sirius, Venus, Vesper,
   celestial body, daystar, evening star, fixed stars, heavenly body,
   living sapphires, lodestar, morning star, orb, polar star,
   polestar, sphere, starry host, stars