plate armor vs helmet

plate armor and helmet both are nouns.

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

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

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

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a historical type of personal armor made from iron or steel plates, which superseded mail during the 14th century.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