The Collaborative International Dictionary of English v.0.48:

Casus \Ca"sus\, n. [L.]
   An event; an occurrence; an occasion; a combination of
   circumstances; a case; an act of God. See the Note under
   Accident.
   [1913 Webster]

   Casus belli, an event or combination of events which is a
      cause war, or may be alleged as a justification of war.

   Casus fortuitus, an accident against which due prudence
      could not have provided. See Act of God, under Act.

   Casus omissus, a case not provided for by the statute.
      [1913 Webster]

WordNet (r) 3.0 (2006):

casus belli
    n 1: an event used to justify starting a war