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