breastplate vs armor plate

breastplate and armor plate both are nouns.

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

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

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

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

breastplate (noun)	armor plate (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