From The Collaborative International Dictionary of English v.0.48:

Upsilon \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.
   [Webster 1913 Suppl.]
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