plate armour vs helmet

plate armour and helmet both are nouns.

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

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

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

plate armour (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
a historical type of personal armor made from iron or steel plates, which superseded mail during the 14th century.	armor plate that protects the head