Source : Webster's Revised Unabridged Dictionary (1913)

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.

Source : WordNet®

upsilon
     n : the 20th letter of the Greek alphabet