Casus

‖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 ]