The Collaborative International Dictionary of English v.0.48:

Zither \Zith"er\, n. [G. zither. See Cittern.] (Mus.)
   An instrument of music used in Austria and Germany. It has
   from thirty to forty wires strung across a shallow
   sounding-board, which lies horizontally on a table before the
   performer, who uses both hands in playing on it.

   Note: [Not to be confounded with the old lute-shaped
         cittern, or cithern.]
         [1913 Webster]

WordNet (r) 3.0 (2006):

zither
    n 1: a musical stringed instrument with strings stretched over a
         flat sounding board; it is laid flat and played with a
         plectrum and with fingers [syn: zither, cither,
         zithern]