plate armour vs coat of mail

plate armour and coat of mail both are nouns.

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

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

plate armour: a historical type of personal armor made from iron or steel plates, which superseded mail during the 14th century.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 armour include body armor, body armour, cataphract, suit of armor, suit of armour.

plate armour (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a historical type of personal armor made from iron or steel plates, which superseded mail during the 14th century.