Roman alphabet vs ABC's

Roman alphabet and ABC's both are nouns.

Word	Noun	Adjective	Verb	Adverb
Roman alphabet	Yes	No	No	No
ABC's	Yes	No	No	No

As nouns, ABC's is a hypernym of Roman alphabet; that is, ABC's is a word with a broader meaning than Roman alphabet:

Roman alphabet: the alphabet evolved by the ancient Romans which serves for writing most of the languages of western Europe
ABC's: a character set that includes letters and is used to write a language

Other hypernyms of Roman alphabet include ABC, ABCs, alphabet, bicameral script.

Roman alphabet (noun)	ABC's (noun)
the alphabet evolved by the ancient Romans which serves for writing most of the languages of western Europe	a character set that includes letters and is used to write a language