ABC's vs letter

ABC's and letter both are nouns.

Word	Noun	Adjective	Verb	Adverb
ABC's	Yes	No	No	No
letter	Yes	No	Yes	No

ABC's (noun)	letter (noun)
a character set that includes letters and is used to write a language	a written message addressed to a person or organization
	the conventional characters of the alphabet used to represent speech
	an award earned by participation in a school sport
	a strictly literal interpretation (as distinct from the intention)
	owner who lets another person use something (housing usually) for hire

ABC's (verb)	letter (verb)
	mark letters on or mark with letters
	set down or print with letters
	win an athletic letter