r vs letter of the alphabet

r and letter of the alphabet both are nouns.

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

As nouns, letter of the alphabet is a hypernym of r; that is, letter of the alphabet is a word with a broader meaning than r:

r: the 18th letter of the Roman alphabet
letter of the alphabet: the conventional characters of the alphabet used to represent speech

Other hypernyms of r include alphabetic character, letter.

r (noun)	letter of the alphabet (noun)
the length of a line segment between the center and circumference of a circle or sphere	the conventional characters of the alphabet used to represent speech
the 18th letter of the Roman alphabet