Etymology
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upsilon (n.)

20th letter of the Greek alphabet, 1640s, from Greek u psilon, literally "a mere (or bare) 'u;' " so called in later Greek in reference to its sound. The exact reason is variously explained, but it seems to have had something to do with distinguishing it from diphthongs.

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Definitions of upsilon

upsilon (n.)
the 20th letter of the Greek alphabet;
From wordnet.princeton.edu