u vs U

u and U both are nouns.

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