plate armour vs greave

plate armour and greave both are nouns.

Word	Noun	Adjective	Verb	Adverb
plate armour	Yes	No	No	No
greave	Yes	No	No	No

As nouns, greave is a hyponym of plate armour; that is, greave is a word with a more specific, narrower meaning than plate armour:

plate armour: specially hardened steel plate used to protect fortifications or vehicles from enemy fire
greave: armor plate that protects legs below the knee

plate armour (noun)	greave (noun)
specially hardened steel plate used to protect fortifications or vehicles from enemy fire	armor plate that protects legs below the knee
a historical type of personal armor made from iron or steel plates, which superseded mail during the 14th century.