helmet vs plate armour

helmet and plate armour both are nouns.

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

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

helmet: armor plate that protects the head
plate armour: 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 armor.

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armor plate that protects the head	a historical type of personal armor made from iron or steel plates, which superseded mail during the 14th century.