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.