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
   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]

Bouvier's Law Dictionary, Revised 6th Ed (1856):

CASUS OMISSUS. An omitted case.
     2. When a statute or an instrument of writing undertakes to foresee and
to provide for certain contingencies, and through mistake, or some other
cause, a case remains to be provided for, it is said to be a casus
omissus.For example, when a statute provides for the descent of intestates
estates, and omits a case, the estate descends as it did before the statute,
whenever that, case occurs, although it appear to be within the general
scope and intent of the statute. 2 Binn. R. 279.
     3. When there has been a casus omissus in a statute, the subject is
ruled by the common law: casus omissuset oblivioni datus dispositioni juris
communis relinquitur. 5 Co. 38. Vide Dig. 38, 1, 44 and 55 Id. 38, 2, 10;
Code, 6, 52, 21 and 30.