- the 20th letter of the Greek alphabet (Υ, υ).
- the vowel sound represented by this letter.
- Also called upsilon particle. Physics. any of a family of heavy, short-lived, neutral mesons that are composed of a bottom quark and its antiquark. Symbol: Υ, υ
- the 20th letter in the Greek alphabet (Υ, υ), a vowel, transliterated as y or u
“20th letter of the Greek alphabet,” 1640s, from Greek u psilon, literally “a mere (or bare) ‘u;’ ” so called in contradistinction to the diphthongs -oi-, -ei-, which are pronounced exactly like upsilon.
- The 20th letter of the Greek alphabet.