\Up"si*lon\, n. [Gr. 'y^ psilo`n bare, mere, simple y.]
The 20th letter ([Upsilon], [upsilon]) 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.
["u]. Its equivalent in English is u or y.