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