helmet vs plate armor

helmet and plate armor both are nouns.

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

As nouns, plate armor is a hypernym of helmet; that is, plate armor is a word with a broader meaning than helmet:

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

Other hypernyms of helmet include armor plate, armor plating, armour plate, plate armour.

helmet (noun)	plate armor (noun)
a protective headgear made of hard material to resist blows	specially hardened steel plate used to protect fortifications or vehicles from enemy fire
armor plate that protects the head	a historical type of personal armor made from iron or steel plates, which superseded mail during the 14th century.