The Collaborative International Dictionary of English v.0.48:

Psaltery \Psal"ter*y\, n.; pl. Psalteries. [OE. sautrie, OF.
   psalterie, F. psalt['e]rion, L. psalterium psaltery, psalter,
   from Gr. ?, fr. ?. See Psalm, Psalter.]
   A stringed instrument of music used by the Hebrews, the form
   of which is not known.
   [1913 Webster]

         Praise the Lord with harp; sing unto him with the
         psaltery and an instrument of ten strings. --Ps.
                                                  xxxiii. 2.
   [1913 Webster]

WordNet (r) 3.0 (2006):

psaltery
    n 1: an ancient stringed instrument similar to the lyre or
         zither but having a trapezoidal sounding board under the
         strings

Easton's 1897 Bible Dictionary:

Psaltery
   a musical instrument, supposed to have been a kind of lyre, or a
   harp with twelve strings. The Hebrew word nebhel, so rendered,
   is translated "viol" in Isa. 5:12 (R.V., "lute"); 14:11. In Dan.
   3:5, 7, 10, 15, the word thus rendered is Chaldaic, pesanterin,
   which is supposed to be a word of Greek origin denoting an
   instrument of the harp kind.