Upsilon (uppercase Υ, lowercase υ; Greek: Ύψιλον, Úpsilon) is the 20th letter of the Greek alphabet. In the system of Greek numerals it has a value of 400. It is derived from the Phoenician waw . The name of the letter is pronounced in Modern Greek, and in English /ˈʌpsɨlɒn/, /juːpˈsaɪlən/ yewp-SY-lən, or /ˈjuːpsɨlɒn/ YEWP-sə-lon. It is occasionally called "ypsilon" (/ˈɪpsɨlɒn/) in English after its rendering in Latin, where the Greek upsilon corresponded with the letter Y.
Read more about Upsilon: Etymology, Pronunciation, Correspondence With Latin Y, Usage, Symbolism