armour plate vs roundel

armour plate and roundel both are nouns.

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

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

armour plate: specially hardened steel plate used to protect fortifications or vehicles from enemy fire
roundel: round piece of armor plate that protects the armpit

armour plate (noun)	roundel (noun)
specially hardened steel plate used to protect fortifications or vehicles from enemy fire	round piece of armor plate that protects the armpit
	(heraldry) a charge in the shape of a filled circle
	English form of rondeau having three triplets with a refrain after the first and third