1. (noun)uracil, U a base containing nitrogen that is found in RNA (but not in DNA) and derived from pyrimidine; pairs with adenine Synonyms: atomic number 92, uranium, uracil Antonyms: low-class, lower-class, middle-class
2. (noun)uranium, U, atomic number 92 a heavy toxic silvery-white radioactive metallic element; occurs in many isotopes; used for nuclear fuels and nuclear weapons Synonyms: atomic number 92, uranium, uracil Antonyms: middle-class, low-class, lower-class
Synonyms and Antonyms of U 
