/ ʌpˈsaɪlən; ˈʊpsɪlɒn; NAmE ˈʊpsɪlɑːn/ noun
the 20th letter of the Greek alphabet (ϒ, υ)
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WORD ORIGIN
Greek , literally slender U , from psilos slender, referring to the need to distinguish upsilon from the diphthong oi : in late Greek the two had the same pronunciation.