The Collaborative International Dictionary of English v.0.48:

Percussion \Per*cus"sion\ (p[~e]r*k[u^]sh"[u^]n), n. [L.
   percussio: cf. F. percussion. See Percuss.]
   1. The act of percussing, or striking one body against
      another; forcible collision, esp. such as gives a sound or
      report. --Sir I. Newton.
      [1913 Webster]

   2. Hence: The effect of violent collision; vibratory shock;
      impression of sound on the ear.
      [1913 Webster]

            The thunderlike percussion of thy sounds. --Shak.
      [1913 Webster]

   3. (Med.) The act of tapping or striking the surface of the
      body in order to learn the condition of the parts beneath
      by the sound emitted or the sensation imparted to the
      fingers. Percussion is said to be immediate if the blow is
      directly upon the body; if some intervening substance, as
      a pleximeter, is, used, it is called mediate.
      [1913 Webster]

   Center of percussion. See under Center.

   Percussion bullet, a bullet containing a substance which is
      exploded by percussion; an explosive bullet.

   Percussion cap, a small copper cap or cup, containing
      fulminating powder, and used with a percussion lock to
      explode gunpowder.

   Percussion fuze. See under Fuze.

   Percussion lock, the lock of a gun that is fired by
      percussion upon fulminating powder.

   Percussion match, a match which ignites by percussion.

   Percussion powder, powder so composed as to ignite by
      slight percussion; fulminating powder.

   Percussion sieve, Percussion table, a machine for sorting
      ores by agitation in running water.
      [1913 Webster]

WordNet (r) 3.0 (2006):

percussion
    n 1: the act of playing a percussion instrument
    2: the act of exploding a percussion cap
    3: the section of a band or orchestra that plays percussion
       instruments [syn: percussion section, percussion, rhythm
       section]
    4: tapping a part of the body for diagnostic purposes [syn:
       percussion, pleximetry]

Moby Thesaurus II by Grady Ward, 1.0:

70 Moby Thesaurus words for "percussion":
   appulse, battery, bells, bones, brunt, bulldozing, bulling, bump,
   cannon, carambole, carom, castanets, celesta, chime, chimes,
   clappers, clash, collision, concussion, crack-up, crash,
   crash cymbal, crump, crunch, cymbals, encounter, finger cymbals,
   gamelan, glockenspiel, gong, hammering, handbells, idiophone,
   impact, impingement, jar, jolt, lyra, maraca, marimba, mauling,
   meeting, metallophone, onslaught, orchestral bells,
   percussion instrument, percussions, percussive, ramming, rattle,
   rattlebones, repercussion, shock, sideswipe, sizzler,
   sledgehammering, smash, smash-up, smashing, snappers, tam-tam,
   thrusting, tintinnabula, tonitruone, triangle, tubular bells,
   vibes, vibraphone, whomp, xylophone