armour plate vs helmet

armour plate and helmet both are nouns.

Word	Noun	Adjective	Verb	Adverb
armour plate	Yes	No	No	No
helmet	Yes	No	No	No

As nouns, helmet is a hyponym of armour plate; that is, helmet is a word with a more specific, narrower meaning than armour plate:

armour plate: specially hardened steel plate used to protect fortifications or vehicles from enemy fire
helmet: armor plate that protects the head

armour plate (noun)	helmet (noun)
specially hardened steel plate used to protect fortifications or vehicles from enemy fire	a protective headgear made of hard material to resist blows
	armor plate that protects the head