The Collaborative International Dictionary of English v.0.48:

Xylophone \Xy"lo*phone\, n. [Xylo- + Gr. fwnh` sound.]
   [1913 Webster]
   1. (Mus.) Originally, an instrument common among the
      Russians, Poles, and Tartars, consisting of a series of
      strips of wood or glass graduated in length to the musical
      scale, resting on belts of straw, and struck with two
      small hammers; now any similar musical instrument composed
      of a series of bars of graduated length. Called in Germany
      strohfiedel, or straw fiddle.
      [1913 Webster +PJC]

   2. An instrument to determine the vibrative properties of
      different kinds of wood. --Knight.
      [1913 Webster]

WordNet (r) 3.0 (2006):

xylophone
    n 1: a percussion instrument with wooden bars tuned to produce a
         chromatic scale and with resonators; played with small
         mallets [syn: marimba, xylophone]

Moby Thesaurus II by Grady Ward, 1.0:

36 Moby Thesaurus words for "xylophone":
   battery, bells, bones, castanets, 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