Princeton's WordNet

1. uracil, U(noun)

   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. uranium, U, atomic number 92(noun)

   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

3. U, u(adj)

   the 21st letter of the Roman alphabet

   Synonyms:
   atomic number 92, uranium, uracil

   Antonyms:
   low-class, middle-class, lower-class

4. u(adj)

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

   Antonyms:
   middle-class, low-class, lower-class