backplate vs plate armor

backplate and plate armor both are nouns.

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

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

backplate: plate armor protecting the back; worn as part of a cuirass
plate armor: specially hardened steel plate used to protect fortifications or vehicles from enemy fire

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

backplate (noun)	plate armor (noun)
plate armor protecting the back; worn as part of a cuirass	specially hardened steel plate used to protect fortifications or vehicles from enemy fire
	a historical type of personal armor made from iron or steel plates, which superseded mail during the 14th century.