U

(noun) a base containing nitrogen that is found in RNA (but not in DNA) and derived from pyrimidine; pairs with adenine. Synonyms: uracil.

(noun) a heavy toxic silvery-white radioactive metallic element; occurs in many isotopes; used for nuclear fuels and nuclear weapons. Synonyms: uranium, atomic number 92.

(noun) the 21st letter of the Roman alphabet. Synonyms: u.

(adj) (chiefly British) of or appropriate to the upper classes especially in language use. Synonyms: u.