Upsilon (uppercase/lowercase Υ υ) is the 20th letter of the Greek alphabet. It is used to represent the "u" sound in Ancient Greek and "i" in Modern Greek. In the system of Greek numerals, it has a value of 400. Letters that came from it include the Roman U, V and Y, and the Cyrillic У.
In mathematics, the uppercase is used to represent the upsilon function (a variant of Riemann zeta function), while the lowercase is used as a general variable. In physics, the uppercase is used to represent an upsilon meson. In astrology, the same letter is used for the sign of Aries.