Source : Webster's Revised Unabridged Dictionary (1913)

Vis major \Vis ma"jor\ [L. major greater.] (Law)
   A superior force which under certain circumstances is held to
   exempt from contract obligations; inevitable accident; -- a
   civil-law term used as nearly equivalent to, but broader
   than, the common-law term {act of God} (which see).

Source : WordNet®

vis major
     n : a natural and unavoidable catastrophe that interrupts the
         expected course of events; "he discovered that his house
         was not insured against acts of God" [syn: {act of God},
         {force majeure}, {inevitable accident}, {unavoidable
         casualty}]