Armor, n.
Etymology: [OE. armure, fr. F. armure, OF. armeure, fr. L. armatura. See Armature.]
[Spelt also armour.]
1. Defensive arms for the body; any clothing or covering worn to protect one's person in battle.
Note: In English statues, armor is used for the whole apparatus of war, including offensive as well as defensive arms. The statues of armor directed what arms every man should provide.
2. Steel or iron covering, whether of ships or forts, protecting them from the fire of artillery. Coat armor, the escutcheon of a person or family, with its several charges and other furniture, as mantling, crest, supporters, motto, etc. -- Submarine, a water-tight dress or covering for a diver. See under Submarine.
Shades of meaning
Fernald has no entry of its own for armor; he lists it under arms.
Where he writes about it:
Arms are implements of attack; armor is a defensive covering. — from arms
The knight put on his armor; he grasped his arms. — from arms
With the disuse of defensive armor the word has practically gone out of military use, but it is still employed in the navy, where the distinction is clearly preserved; any vessel provided with cannon is an armed vessel; an armored ship is an ironclad. — from arms