Source : Webster's Revised Unabridged Dictionary (1913)

Coma \Co"ma\ (k[=o]"m[.a]), n. [NL., fr. Gr. kw^ma lethargy, fr.
   koima^n to put to sleep. See {Cemetery}.]
   A state of profound insensibility from which it is difficult
   or impossible to rouse a person. See {Carus}.

Coma \Co"ma\, n. [L., hair, fr. Gr. ko`mh.]
   1. (Astron.) The envelope of a comet; a nebulous covering,
      which surrounds the nucleus or body of a comet.

   2. (Bot.) A tuft or bunch, -- as the assemblage of branches
      forming the head of a tree; or a cluster of bracts when
      empty and terminating the inflorescence of a plant; or a
      tuft of long hairs on certain seeds.

   {Coma Berenices}[L.] (Astron.), a small constellation north
      of Virgo; -- called also {Berenice's Hair}.

Source : WordNet®

coma
     n 1: a state of deep and often prolonged unconsciousness; usually
          the result of disease or injury [syn: {comatoseness}]
     2: a usually terminal tuft of hairs especially on a seed
     3: (astronomy) the luminous cloud of particles surrounding the
        frozen nucleus of a comet; forms as the comet approaches
        the sun and is warmed
     [also: {comae} (pl)]