Roman numeral vs numeral

Roman numeral and numeral both are nouns.

Word	Noun	Adjective	Verb	Adverb
Roman numeral	Yes	No	No	No
numeral	Yes	Yes	No	No

As nouns, numeral is a hypernym of Roman numeral; that is, numeral is a word with a broader meaning than Roman numeral:

Roman numeral: a symbol in the old Roman notation; I,V,X,L,C,D,M represent 1, 5, 10, 50, 100, 500, 1000 respectively in Arabic notation
numeral: a symbol used to represent a number

Other hypernyms of Roman numeral include number.

Roman numeral (noun)	numeral (noun)
a symbol in the old Roman notation; I,V,X,L,C,D,M represent 1, 5, 10, 50, 100, 500, 1000 respectively in Arabic notation	a symbol used to represent a number
	a word expressing a number.

Roman numeral (adjective)	numeral (adjective)
	of or relating to or denoting numbers