plastron vs breastplate

plastron and breastplate both are nouns.

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

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

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

Other hypernyms of plastron include aegis, egis.

plastron (noun)	breastplate (noun)
(zoology) the part of a turtle's shell forming its underside	armor plate that protects the chest; the front part of a cuirass
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