letter vs ABC

letter and ABC both are nouns.

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

letter (noun)	ABC (noun)
a written message addressed to a person or organization	a character set that includes letters and is used to write a language
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

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