NOUN (2)

1. injury to the brain caused by a blow; usually resulting in loss of consciousness;

2. any violent blow;

The Collaborative International Dictionary of English v.0.48:

Concussion \Con*cus"sion\, n. [L. concussio, fr. concutere,
   concussum, to shake violenty; con- + quatere to shake. See
   Cashier, Quash.]
   1. A shaking or agitation; a shock; caused by the collision
      of two bodies.
      [1913 Webster]

            It is believed that great ringing of bells, in
            populous cities, hath dissipated pestilent air;
            which may be from the concussion of the air.
                                                  --Bacon.
      [1913 Webster]

   2. (Med.) A condition of lowered functional activity, without
      visible structural change, produced in an organ by a
      shock, as by fall or blow; as, a concussion of the brain.
      [1913 Webster]

   3. (Civil Law) The unlawful forcing of another by threats of
      violence to yield up something of value.
      [1913 Webster]

            Then concussion, rapine, pilleries,
            Their catalogue of accusations fill.  --Daniel.
      [1913 Webster]

   Concussion fuse (Mil.), one that is ignited by the
      concussion of the shell when it strikes.

   Syn: See Shock.
        [1913 Webster]

WordNet (r) 3.0 (2006):

concussion
    n 1: injury to the brain caused by a blow; usually resulting in
         loss of consciousness
    2: any violent blow

Moby Thesaurus II by Grady Ward, 1.0:

88 Moby Thesaurus words for "concussion":
   abrasion, appulse, beating, blemish, blow, break, brunt, buffeting,
   bulldozing, bulling, bump, burn, cannon, carambole, carom, chafe,
   check, chip, clash, clip, clout, collision, crack, crack-up,
   crackle, crash, craze, crump, crunch, cut, encounter, flash burn,
   fracture, fray, frazzle, gall, gash, hammering, hurt, impact,
   impingement, incision, injury, jar, jarring, jolt, jolting,
   laceration, lesion, mauling, meeting, mortal wound, mutilation,
   onslaught, percussion, pounding, puncture, ramming, rent,
   repercussion, rip, run, rupture, scald, scorch, scrape, scratch,
   scuff, second-degree burn, shaking, shock, sideswipe, slash,
   sledgehammering, smash, smash-up, smashing, sore, stab, stab wound,
   tear, third-degree burn, thrusting, trauma, whomp, wound,
   wounds immedicable, wrench



Bouvier's Law Dictionary, Revised 6th Ed (1856):

CONCUSSION, civ. law. The unlawful forcing of another by threats of violence
to give something of value. It differs from robbery in this, that in robbery
the thing is taken by force, while in concussion it is obtained by
threatened violence. Hein. Lec. El, Sec. 1071