Greek alphabet vs ABC's

Greek alphabet and ABC's both are nouns.

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

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

Greek alphabet: the alphabet used by ancient Greeks
ABC's: a character set that includes letters and is used to write a language

Other hypernyms of Greek alphabet include ABC, ABCs, alphabet.

Greek alphabet (noun)	ABC's (noun)
the alphabet used by ancient Greeks	a character set that includes letters and is used to write a language