U vs letter of the alphabet

U and letter of the alphabet both are nouns.

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

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

U: the 21st letter of the Roman alphabet
letter of the alphabet: the conventional characters of the alphabet used to represent speech

Other hypernyms of U include alphabetic character, letter.

U (noun)	letter of the alphabet (noun)
the 21st letter of the Roman alphabet	the conventional characters of the alphabet used to represent speech
a heavy toxic silvery-white radioactive metallic element; occurs in many isotopes; used for nuclear fuels and nuclear weapons
a base containing nitrogen that is found in RNA (but not in DNA) and derived from pyrimidine; pairs with adenine