NOUN (2)

1. a stringed instrument used in American folk music; an elliptical body and a fretted fingerboard and three strings;

2. a trapezoidal zither whose metal strings are struck with light hammers;

The Collaborative International Dictionary of English v.0.48:

Dulcimer \Dul"ci*mer\, n. [It. dolcemele,r Sp. dulcemele, fr. L.
   dulcis sweet + melos song, melody, Gr. ?; cf. OF. doulcemele.
   See Dulcet, and Melody.] (Mus.)
   (a) An instrument, having stretched metallic wires which are
       beaten with two light hammers held in the hands of the
       performer.
   (b) An ancient musical instrument in use among the Jews.
       --Dan. iii. 5. It is supposed to be the same with the
       psaltery.
       [1913 Webster]

WordNet (r) 3.0 (2006):

dulcimer
    n 1: a stringed instrument used in American folk music; an
         elliptical body and a fretted fingerboard and three strings
    2: a trapezoidal zither whose metal strings are struck with
       light hammers

Easton's 1897 Bible Dictionary:

Dulcimer
   (Heb. sumphoniah), a musical instrument mentioned in Dan. 3:5,
   15, along with other instruments there named, as sounded before
   the golden image. It was not a Jewish instrument. In the margin
   of the Revised Version it is styled the "bag-pipe." Luther
   translated it "lute," and Grotius the "crooked trumpet." It is
   probable that it was introduced into Babylon by some Greek or
   Western-Asiatic musician. Some Rabbinical commentators render it
   by "organ," the well-known instrument composed of a series of
   pipes, others by "lyre." The most probable interpretation is
   that it was a bag-pipe similar to the zampagna of Southern
   Europe.