What is the 20th letter (U, u) 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. ü. Its equivalent in English is u or y called?

upsilon — The 20th letter (U, u) 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. ü. Its equivalent in English is u or y. (Webster's 1913.)