Definition of upsilon noun from the Oxford Advanced Learner's Dictionary

 

upsilon

 noun
noun
BrE BrE//ʌpˈsaɪlən//
 
, BrE//ˈʊpsɪlɒn//
 
; NAmE NAmE//ˈʊpsɪlɑːn//
 
 
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the 20th letter of the Greek alphabet (ϒ, υ) Word OriginGreek, 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.
See the Oxford Advanced American Dictionary entry: upsilon