Source : Webster's Revised Unabridged Dictionary (1913)

U \U\ ([=u]),
   the twenty-first letter of the English alphabet, is a cursive
   form of the letter V, with which it was formerly used
   interchangeably, both letters being then used both as vowels
   and consonants. U and V are now, however, differentiated, U
   being used only as a vowel or semivowel, and V only as a
   consonant. The true primary vowel sound of U, in Anglo-Saxon,
   was the sound which it still retains in most of the languages
   of Europe, that of long oo, as in tool, and short oo, as in
   wood, answering to the French ou in tour. Etymologically U is
   most closely related to o, y (vowel), w, and v; as in two,
   duet, dyad, twice; top, tuft; sop, sup; auspice, aviary. See
   V, also O and Y. See Guide to Pronunciation, [sect][sect]
   130-144.

Source : WordNet®

U
     n 1: a nitrogen-containing base found in RNA (but not in DNA) and
          derived from pyrimidine; pairs with adenine [syn: {uracil}]
     2: a heavy toxic silvery-white radioactive metallic element;
        occurs in many isotopes; used for nuclear fuels and
        nuclear weapons [syn: {uranium}, {atomic number 92}]
     3: the 21st letter of the Roman alphabet