upsilon

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up·si·lon / ˈəpsəˌlän; ˈ(y)oōp-/ • n. the twentieth letter of the Greek alphabet (Υ, υ), transliterated in the traditional Latin style as ‘y’ (as in cycle) or in the modern style as ‘u’ (as in the etymologies of this dictionary). ∎  (Upsilon) [followed by Latin genitive] Astron. the twentieth star in a constellation: Upsilon Scorpii. ∎  (also up·si·lon par·ti·cle) Physics a meson thought to contain a b quark bound to its antiparticle, produced in particle accelerators.

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