Arma is originally a Latin word:
arma (genitive armōrum); n pl, second declension (plural only; not used in singular form)
1. defensive
arms, armor/armour, shields, weapons of war.
2. war
3. soldiers, military
power4. defence
5. tools
ArmA is replacement name of
games successing Operation Flashpoint (name Operation Flashpoint belongs to publisher Codemasters ) so ArmA 1 is Flashpoint 2 that cannot be called Flashpoint, same goes for ArmA 2 and 3.
ArmA is also originally meant as acronym of Armed Assault (used only in
context of ArmA (1): Armed Assault )
ArmA 3 is ultimate open-world war simulation game by czech game developers Bohemia Interactive