XXI vs twenty-one

XXI and twenty-one both are nouns.

Word	Noun	Adjective	Verb	Adverb
XXI	Yes	No	No	No
twenty-one	Yes	Yes	No	No

As nouns, XXI and twenty-one are synonyms defined as:

XXI and twenty-one: the cardinal number that is the sum of twenty and one

Other synonyms of XXI include 21.

XXI (noun)	twenty-one (noun)
the cardinal number that is the sum of twenty and one	a gambling game using cards; the object is to hold cards having a higher count than those dealt to the banker up to but not exceeding 21
	the cardinal number that is the sum of twenty and one

XXI (adjective)	twenty-one (adjective)
	being one more than twenty