Pronunciation: (yOOp'su-lon", -lun, up'-; esp. Brit. yOOp-sī'lun),[key] —n. 1. the 20th letter of the Greek alphabet (Υ, υ).
2. the vowel sound represented by this letter.
3. Also called up'silon par"ticle,Physics.any of a family of heavy, short-lived, neutral mesons that are composed of a bottom quark and its antiquark. Symbol: Υ, υ