What does "casus" mean?

Casus: 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.