[ ʌp'sʌɪlən , ju:p- , 'ʊpsɪlɒn , 'ju:p- ]
■ noun the twentieth letter of the Greek alphabet ( Υ , υ ), transliterated as ‘u’ or (chiefly in English words derived through Latin) as ‘y’.
Origin
Gk, lit. 'slender U', from psilos 'slender', referring to the need to distinguish ~ from the diphthong oi : in late Gk the two had the same pronunciation.