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