twenty-one vs XXI

twenty-one and XXI both are nouns.

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

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

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

Other synonyms of twenty-one include 21.

twenty-one (noun)	XXI (noun)
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
the cardinal number that is the sum of twenty and one

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