Armageddon = "the hill or city of Megiddo"
1) In Rev 16:16 the scene of a the struggle of good and evil is suggested by that battle plain of Esdraelon, which was famous for two great victories, of Barak over the Canaanites, and of Gideon over the Midianites; and for two great disasters, the deaths of Saul and Josiah. Hence in Revelation a place of great slaughter, the scene of a terrible retribution upon the wicked. The RSV translates the name as Har-Magedon, i.e. the hill (as Ar is the city) of Megiddo.
Part of Speech: noun proper locative
Citing in TDNT: 1:468, 79
Of Hebrew origin [H2022] and [H4023]; Armageddon (or Har-Megiddon), a symbolical name: - Armageddon.
These two sites give similar information, with the definition from several dictionaries and statistics on the use of the word.
Perseus Digital Library
University of Chicago's Logion lexicon
Thayer's dictionary plus other information.
From this site's dictionary (in Italian)
Click on the first column to search for that word as a form of the root Ἁρμαγεδών; click on the second column to search for that grammatical form of the root Ἁρμαγεδών; click on the third column to search for that word and grammatical form; click on the total to search for the root Ἁρμαγεδών.