U vs u

U and u both are nouns.

Word	Noun	Adjective	Verb	Adverb
U	Yes	No	No	No
u	Yes	Yes	No	No

As nouns, U and u are synonyms defined as:

U (noun)	u (noun)
the 21st letter of the Roman alphabet	the 21st letter of the Roman alphabet
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

U (adjective)	u (adjective)
	(chiefly British) of or appropriate to the upper classes especially in language use