The Collaborative International Dictionary of English v.0.48:

Castanets \Cas"ta*nets\, n. pl. [F. castagnettes, Sp.
   casta[~n]etas, fr. L. castanea (Sp. casta[~n]a) a chestnut.
   So named from the resemblance to two chestnuts, or because
   chestnuts were first used for castanets. See Chestnut.]
   Two small, concave shells of ivory or hard wood, shaped like
   spoons, fastened to the thumb, and beaten together with the
   middle finger; -- used by the Spaniards and Moors as an
   accompaniment to their dance and guitars.
   [1913 Webster]

   Note: The singular, castanet, is used of one of the pair, or,
         sometimes, of the pair forming the instrument.
         [1913 Webster]

               The dancer, holding a castanet in each hand,
               rattles them to the motion of his feet. --Moore
                                                  (Encyc. of
                                                  Music).
         [1913 Webster]

WordNet (r) 3.0 (2006):

castanets
    n 1: a percussion instrument consisting of a pair of hollow
         pieces of wood or bone (usually held between the thumb and
         fingers) that are made to click together (as by Spanish
         dancers) in rhythm with the dance [syn: bones,
         castanets, clappers, finger cymbals]

Moby Thesaurus II by Grady Ward, 1.0:

36 Moby Thesaurus words for "castanets":
   battery, bells, bones, celesta, chime, chimes, clappers,
   crash cymbal, cymbals, finger cymbals, gamelan, glockenspiel, gong,
   handbells, idiophone, lyra, maraca, marimba, metallophone,
   orchestral bells, percussion, percussion instrument, percussions,
   percussive, rattle, rattlebones, sizzler, snappers, tam-tam,
   tintinnabula, tonitruone, triangle, tubular bells, vibes,
   vibraphone, xylophone