alphabet vs letter of the alphabet

alphabet and letter of the alphabet both are nouns.

Word	Noun	Adjective	Verb	Adverb
alphabet	Yes	No	No	No
letter of the alphabet	Yes	No	No	No

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