breastplate vs plate armour

breastplate and plate armour both are nouns.

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

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

breastplate: armor plate that protects the chest; the front part of a cuirass
plate armour: 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 armor.

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	a historical type of personal armor made from iron or steel plates, which superseded mail during the 14th century.