The Collaborative International Dictionary of English v.0.48:

Guitar \Gui*tar"\, n. [F. guitare; cf. Pr., Sp., & Pg.guitarra,
   It. chitarra; all fr. Gr. ?; cf. L. cithara. Cf. Cittern,
   Gittern.]
   A stringed instrument of music resembling the lute or the
   violin, but larger, and having six strings, three of silk
   covered with silver wire, and three of catgut, -- played upon
   with the fingers.
   [1913 Webster]

WordNet (r) 3.0 (2006):

guitar
    n 1: a stringed instrument usually having six strings; played by
         strumming or plucking

Moby Thesaurus II by Grady Ward, 1.0:

33 Moby Thesaurus words for "guitar":
   Dobro guitar, F-hole guitar, Spanish guitar, archlute, balalaika,
   banjo, banjo-uke, banjo-ukulele, banjo-zither, banjorine, banjuke,
   bass guitar, centerhole guitar, concert guitar, electric guitar,
   lute, mando-bass, mando-cello, mandola, mandolin, mandolute,
   mandore, oud, pandora, pandura, samisen, sitar, steel guitar,
   tamboura, theorbo, troubadour fiddle, uke, ukulele