upsilon

[yoop-suh-lon, -luhn, uhp-; especially British yoop-sahy-luhn]
noun
1.
the 20th letter of the greek alphabet (Υ, υ).
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. Symbol: Υ, υ

Origin:
1615–25; < Late Greek ŷ psīlón literally, simple u (to distinguish it from the digraph oi, pronounced the same in LGk)

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World English Dictionary
upsilon (ˈʌpsɪˌlɒn, juːpˈsaɪlən)
 
n
the 20th letter in the Greek alphabet (Υ, υ), a vowel, transliterated as y or u
 
[C17: from Medieval Greek u psilon simple u, name adopted for graphic u to avoid confusion with graphic oi, since pronunciation was the same for both in Late Greek]

Collins English Dictionary - Complete & Unabridged 10th Edition
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Etymonline
Word Origin & History

upsilon
"20th letter of the Gk. alphabet," 1642, from Gk. u psilon, lit. "a mere (or bare) 'u;' " so called in contradistinction to the diphthongs -oi-, -ei-, which are pronounced exactly like upsilon.
Online Etymology Dictionary, © 2010 Douglas Harper
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American Heritage
Medical Dictionary

upsilon up·si·lon or yp·si·lon (ŭp'sə-lŏn', yōōp'-)
n.

Symbol υ The 20th letter of the Greek alphabet.

The American Heritage® Stedman's Medical Dictionary
Copyright © 2002, 2001, 1995 by Houghton Mifflin Company. Published by Houghton Mifflin Company.
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Example sentences
Recently researchers have been able to describe the electrophysiology of the upsilon cell.
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