'Upsilon' definitions:

Definition of 'upsilon'

(from WordNet)
noun
The 20th letter of the Greek alphabet

Definition of 'Upsilon'

From: GCIDE
  • 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.]