Up"si*lon (?), n. [Gr. 'y^ psilo`n bare, mere, simple y.]
The 20th letter (Υ, υ) of the Greek alphabet, a vowel having originally the sound of OO as in room, becoming before the 4th century b. c. that French u or Ger. ü. Its equivalent in English is u or y.
© Webster 1913