plastron vs aegis

plastron and aegis both are nouns.

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

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

Other hypernyms of plastron include breastplate, egis.

plastron (noun)	aegis (noun)
(zoology) the part of a turtle's shell forming its underside	kindly endorsement and guidance
a large pad worn by a fencer to protect the chest	armor plate that protects the chest; the front part of a cuirass
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