backplate vs armor plate

backplate and armor plate both are nouns.

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

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

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

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

backplate (noun)	armor plate (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