The word Armageddon is found only once in the Bible, in Revelation 16:16.  “And He gathered them together at the place called in Hebrew Harmagedōn.”

There is a great variety of spellings of the word in the Greek and Latin manuscripts of the New Testament.  But there are two main divisions of the spellings: those with just Magedon, and those with the AR in front.

The earliest Greek manuscripts have αρμαγεδων.  Here is the image snipped from Codex Alexandrinus (Codex A), 5th century:

The Greek New Testament editions read as follows:

Ἁρμαγεδών  Antoniades, Robinson-Pierpont, SBL, and NA28

Ἀρμαγεδών  BG (Byzantine Greek), and TH (Tyndale House)

Ἀρμαγεδδών  TR (Textus Receptus)

The difference between the first two spellings is the “breathing mark” in front of or above the initial vowel.  The first one has a backwards apostrophe, and that is the “h” sound in Greek, so Harmagedon.”  The second and third spellings have a regular-facing apostrophe and so that has no “h” sound, thus Armagedon.

John tells us that it is a Hebrew name, so the Har would be Hebrew for mountain, and Magedō would be the place called Megiddo, mentioned in 2 Chronicles 35:22 and Judges 1:27.  So “Mountain of Megiddō.”  This was a frequent battleground because of a strategic pass and the Megiddo plain below. You can download here the book of Revelation with a fuller accounting of the readings in the Greek manuscripts.