The Collaborative International Dictionary of English v.0.48:

Trombone \Trom"bone\, n. [It., aug. of tromba a trumpet: cf. F.
   trombone. See Trump a trumpet.]
   1. (Mus.) A powerful brass instrument of the trumpet kind,
      thought by some to be the ancient sackbut, consisting of a
      tube in three parts, bent twice upon itself and ending in
      a bell. The middle part, bent double, slips into the outer
      parts, as in a telescope, so that by change of the
      vibrating length any tone within the compass of the
      instrument (which may be bass or tenor or alto or even, in
      rare instances, soprano) is commanded. It is the only
      member of the family of wind instruments whose scale, both
      diatonic and chromatic, is complete without the aid of
      keys or pistons, and which can slide from note to note as
      smoothly as the human voice or a violin. Softly blown, it
      has a rich and mellow sound, which becomes harsh and
      blatant when the tones are forced; used with discretion,
      its effect is often solemn and majestic.
      [1913 Webster]

   2. (Zool.) The common European bittern.
      [1913 Webster]

WordNet (r) 3.0 (2006):

trombone
    n 1: a brass instrument consisting of a long tube whose length
         can be varied by a U-shaped slide

Moby Thesaurus II by Grady Ward, 1.0:

102 Moby Thesaurus words for "trombone":
   English horn, alpenhorn, alphorn, althorn, alto horn, ballad horn,
   baritone, bass horn, bassoon, block flute, bombard, bourdon,
   brass choir, brass wind, brass-wind instrument, brasses, bugle,
   bugle horn, cello, claribel, clarinet, clarion, concert flute,
   cornet, cornet-a-pistons, corno di caccia, cornopean, cromorna,
   cymbel, diapason, double-bell euphonium, dulciana, euphonium,
   flute stop, foundation stop, fourniture, gamba, gedeckt, gemshorn,
   harmonic flute, helicon, horn, hunting horn, hybrid stop,
   key trumpet, koppel flute, larigot, lituus, lur, mellophone,
   melodia, mixture, mutation stop, nazard, oboe, octave, ophicleide,
   orchestral horn, organ stop, piccolo, plein jeu, pocket trumpet,
   posaune, post horn, principal, quint, quintaten, rank, ranket,
   reed stop, register, rohr flute, sackbut, saxhorn, saxtuba,
   serpent, sesquialtera, shawm, slide trombone, sliphorn, sousaphone,
   spitz flute, stop, stopped diapason, stopped flute,
   string diapason, string stop, tenor tuba, tierce, tremolo, tromba,
   trumpet, tuba, twelfth, unda maris, valve trombone, valve trumpet,
   vibrato, viola, voix celeste, vox angelica, vox humana