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
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.
Bouvier's Law Dictionary, Revised 6th Ed (1856):
CASUS FORTUITUS. A fortuitous case; an uncontrollable accident an act of
God. See Act of God; Cas fortuit; Fortuitous event.