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"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.
upsilon up·si·lon or yp·si·lon (ŭp'sə-lŏn', yōōp'-)
Symbol υ The 20th letter of the Greek alphabet.