plate armor vs coat of mail

plate armor and coat of mail both are nouns.

Word	Noun	Adjective	Verb	Adverb
plate armor	Yes	No	No	No
coat of mail	Yes	No	No	No

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

plate armor: a historical type of personal armor made from iron or steel plates, which superseded mail during the 14th century.
coat of mail: armor that protects the wearer's whole body

Other hypernyms of plate armor include body armor, body armour, cataphract, suit of armor, suit of armour.

plate armor (noun)	coat of mail (noun)
specially hardened steel plate used to protect fortifications or vehicles from enemy fire	armor that protects the wearer's whole body
a historical type of personal armor made from iron or steel plates, which superseded mail during the 14th century.