ABC vs alphabet

ABC and alphabet both are nouns.

Word	Noun	Adjective	Verb	Adverb
ABC	Yes	No	No	No
alphabet	Yes	No	No	No

As nouns, ABC and alphabet are synonyms defined as:

ABC and alphabet: a character set that includes letters and is used to write a language

Other synonyms of ABC include ABC's, ABCs.

ABC (noun)	alphabet (noun)
a character set that includes letters and is used to write a language	the elementary stages of any subject (usually plural)
	a character set that includes letters and is used to write a language