upsilon Play nounThe 20th letter of the Greek alphabet.nounThe Greek letter γ, υ, corresponding to the English u (and y).nounThe Greek letter Γ, υ, corresponding to the English u (and y).nounThe 20th letter (Υ, υ) of the Greek alphabet, a vowel having originally the sound of � as in room, becoming before the 4th century b. c. that French u or Ger. ü. Its equivalent in English is u or y.nounThe twentieth letter of the Classical and Modern Greek, the twenty-first letter of Old and Ancient.nounthe 20th letter of the Greek alphabet