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"

Easton's 1897 Bible Dictionary:

Armageddon
   occurs only in Rev. 16:16 (R.V., "Har-Magedon"), as symbolically
   designating the place where the "battle of that great day of God
   Almighty" (ver. 14) shall be fought. The word properly means the
   "mount of Megiddo." It is the scene of the final conflict
   between Christ and Antichrist. The idea of such a scene was
   suggested by the Old Testament great battle-field, the plain of
   Esdraelon (q.v.).


Hitchcock's Bible Names Dictionary (late 1800's):

Armageddon, hill of fruits; mountain of Megiddo