The Collaborative International Dictionary of English v.0.48:

Gittith \Git"tith\ (g[i^]t"t[i^]th), n. [Heb.]
   A musical instrument, of unknown character, supposed by some
   to have been used by the people of Gath, and thence obtained
   by David. It is mentioned in the title of Psalms viii.,
   lxxxi., and lxxxiv. --Dr. W. Smith.
   [1913 Webster]

Easton's 1897 Bible Dictionary:

Gittith
   a stringed instrument of music. This word is found in the titles
   of Ps. 8, 81, 84. In these places the LXX. render the word by
   "on the wine-fats." The Targum explains by "on the harp which
   David brought from Gath." It is the only stringed instrument
   named in the titles of the Psalms.