Source : Webster's Revised Unabridged Dictionary (1913)

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}.

   {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.

Source : WordNet®

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