backplate vs plate armour

backplate and plate armour both are nouns.

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

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

backplate: plate armor protecting the back; worn as part of a cuirass
plate armour: 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 armor.

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	a historical type of personal armor made from iron or steel plates, which superseded mail during the 14th century.