/yoohp"seuh lon', -leuhn, up"-/ ; esp. Brit. /yoohp suy"leuhn/ , n.
1. the 20th letter of the Greek alphabet (Y).
2. the vowel sound represented by this letter.
3. 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. Symbolically represented by the greek letter Upsilon.
[ 1615-25; y psilón lit., simple u (to distinguish it from the digraph oi, pronounced the same in LGk) ]