The Collaborative International Dictionary of English v.0.48:

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).
   [Webster 1913 Suppl.]

WordNet (r) 3.0 (2006):

vis major
    n 1: 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, vis major, inevitable accident,
         unavoidable casualty]