The Collaborative International Dictionary of English v.0.48:

cither \cither\ n.
   a musical instrument resembling a harp with strings stretch
   over a flat sounding box; the zither. It is played with a
   plectrum and with fingers.

   Syn: zither, zithern.
        [WordNet 1.5]

WordNet (r) 3.0 (2006):

cither
    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]
    2: a 16th century musical instrument resembling a guitar with a
       pear-shaped soundbox and wire strings [syn: cittern,
       cithern, cither, citole, gittern]