Upsilon

Up″si‐lon (?), n. [Gr. ὐ̑ ψιλόν bare, mere, simple y.] The 20th letter (Υ, υ) of the Greek alphabet, a vowel having originally the sound of o͞o as in room, becoming before the 4th century b. c. that French u or Ger. ü. Its equivalent in English is u or y.