The Collaborative International Dictionary of English v.0.48:

Mustache \Mus*tache"\ (m[u^]s*t[.a]sh"; 277), n.; pl.
   Mustaches. [Written also moustache.] [F. moustache, It.
   mostaccio visage, mostacchio mustache, fr. Gr. my`stax upper
   lip and the beard upon it; cf. ma`stax mouth: cf. Sp.
   mostacho.]
   1. That part of the beard which grows on the upper lip; hair
      left growing above the mouth.
      [1913 Webster]

   2. (Zool.) A West African monkey (Cercopithecus cephus). It
      has yellow whiskers, and a triangular blue mark on the
      nose.
      [1913 Webster]

   3. (Zool.) Any conspicuous stripe of color on the side of the
      head, beneath the eye of a bird.
      [1913 Webster]

   4. A stain or discoloration on the upper lip of a person; as,
      wearing a milk mustache. [informal]
      [PJC]

WordNet (r) 3.0 (2006):

mustache
    n 1: an unshaved growth of hair on the upper lip; "he looked
         younger after he shaved off his mustache" [syn: mustache,
         moustache]