breastplate vs plastron

breastplate and plastron both are nouns.

Word	Noun	Adjective	Verb	Adverb
breastplate	Yes	No	No	No
plastron	Yes	No	No	No

As nouns, plastron is a hyponym of breastplate; that is, plastron is a word with a more specific, narrower meaning than breastplate:

breastplate: armor plate that protects the chest; the front part of a cuirass
plastron: a metal breastplate that was worn under a coat of mail

breastplate (noun)	plastron (noun)
armor plate that protects the chest; the front part of a cuirass	(zoology) the part of a turtle's shell forming its underside
	a large pad worn by a fencer to protect the chest
	a metal breastplate that was worn under a coat of mail
	the front of man's dress shirt
	the ornamental front of a woman's bodice or shirt