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]

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

VIS MAJOR, a superior force. In law it signifies inevitable accident.
     2. This term is used in the civil law in nearly the same same way that
the words act of God, (q.v.) are used in the common law. Generally, no one
is responsible for an accident which arises from the vis major; but a man
may be so where he has stipulated that he would; and when he has been guilty
of a fraud or deceit. 2 Kent, Com. 448; Poth. Pret a Usage, n. 48, n. 60
Story Bailm. Sec. 25.