The Collaborative International Dictionary of English v.0.48:

Misadventure \Mis`ad*ven"ture\ (?; 135), n. [OE. mesaventure, F.
   m['e]saventure.]
   Mischance; misfortune; ill luck; unlucky accident; ill
   adventure. --Chaucer.
   [1913 Webster]

   Homicide by misadventure (Law), homicide which occurs when
      a man, doing a lawful act, without any intention of
      injury, unfortunately kills another; -- called also
      excusable homicide. See Homicide. --Blackstone.
      [1913 Webster]

   Syn: Mischance; mishap; misfortune; disaster; calamity.
        [1913 Webster]

WordNet (r) 3.0 (2006):

misadventure
    n 1: an instance of misfortune [syn: mishap, misadventure,
         mischance]

Moby Thesaurus II by Grady Ward, 1.0:

33 Moby Thesaurus words for "misadventure":
   accident, blow, blunder, boner, bull, calamity, casualty,
   cataclysm, catastrophe, collision, contretemps, crack-up, crash,
   disaster, error, faux pas, grief, howler, ill hap, lapse,
   mischance, misfortune, mishap, nasty blow, pileup, shipwreck,
   shock, slip, smash, smashup, staggering blow, tragedy, wreck



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

MISADVENTURE, crim. law, torts. An accident by which an injury occurs to
another.
     2. When applied to homicide, misadventure is the act of a man who, in
the performance of a lawful act, without any intention to do harm, and after
using proper precaution to prevent danger, unfortunately kills another
person. The act upon which the death ensues, must be neither malum in se,
nor malum prohibitum. The usual examples under this head are, 1. When the
death ensues from innocent recreations. 2. From moderate and lawful
correction (q.v.)  in foro domestico. 3. From acts lawful and indifferent
in themselves, done with proper and ordinary caution. 4 Bl. Com. 182; 1
East, P C. 221.