The Collaborative International Dictionary of English v.0.48:

Casualty \Cas"u*al*ty\, n.; pl. Casualties. [F. casualit['e],
   LL. casualitas.]
   1. That which comes without design or without being foreseen;
      contingency.
      [1913 Webster]

            Losses that befall them by mere casualty. --Sir W.
                                                  Raleigh.
      [1913 Webster]

   2. Any injury of the body from accident; hence, death, or
      other misfortune, occasioned by an accident; as, an
      unhappy casualty.
      [1913 Webster]

   3. pl. (Mil. & Naval) Numerical loss caused by death, wounds,
      discharge, or desertion.
      [1913 Webster]

   Casualty ward, A ward in a hospital devoted to the
      treatment of injuries received by accident.

   Syn: Accident; contingency; fortuity; misfortune.
        [1913 Webster]

WordNet (r) 3.0 (2006):

casualty
    n 1: someone injured or killed or captured or missing in a
         military engagement
    2: someone injured or killed in an accident [syn: casualty,
       injured party]
    3: an accident that causes someone to die [syn: fatal
       accident, casualty]
    4: a decrease of military personnel or equipment