breastplate vs plate armor

breastplate and plate armor both are nouns.

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

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

breastplate: armor plate that protects the chest; the front part of a cuirass
plate armor: specially hardened steel plate used to protect fortifications or vehicles from enemy fire

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

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	a historical type of personal armor made from iron or steel plates, which superseded mail during the 14th century.