From WordNet (r) 3.0 (2006) [wn]:
guitar
n 1: a stringed instrument usually having six strings; played by
strumming or plucking
From The Collaborative International Dictionary of English v.0.48 [gcide]:
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]