P vs letter of the alphabet

P and letter of the alphabet both are nouns.

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

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

P: the 16th letter of the Roman alphabet
letter of the alphabet: the conventional characters of the alphabet used to represent speech

Other hypernyms of P include alphabetic character, letter.

P (noun)	letter of the alphabet (noun)
the 16th letter of the Roman alphabet	the conventional characters of the alphabet used to represent speech
a multivalent nonmetallic element of the nitrogen family that occurs commonly in inorganic phosphate rocks and as organic phosphates in all living cells; is highly reactive and occurs in several allotropic forms