noun pl. U's, u's

1. the twenty-first letter of the English alphabet: formerly a variant of V, v; not until the 18th cent. was it established as a vowel symbol only
2. any of the speech sounds that this letter represents, as, in English, the vowel (u) of cut, (o̵o) of bush, or (o̵̅o̅) of rude, or the semivowel (w) of win
3. a type or impression for u or U
4. the twenty-first in a sequence or group
5. an object shaped like

adjective

1. of u or U
2. twenty-first in a sequence or group
3. shaped like