The Collaborative International Dictionary of English v.0.48:

Armageddon \Ar`ma*ged"don\ ([aum]r*m[.a]"j[e^]"d'n), n.
   the final, decisive battle between the forces of good and
   evil, as foretold in the Apocolypse of Saint John. Also, the
   site of that battle. Used metaphorically for a vast and
   decisive conflict, attended by cataclysmic destruction.
   [PJC]

WordNet (r) 3.0 (2006):

Armageddon
    n 1: (New Testament) the scene of the final battle between the
         kings of the Earth at the end of the world
    2: any catastrophically destructive battle; "they called the
       first World War an Armageddon"