Xi (uppercase/lowercase Ξ ξ) is the 14th letter of the Greek alphabet. It is used to represent the "x" sound in Ancient and Modern Greek. In the system of Greek numerals, it has a value of 60. The Roman letter X and Cyrillic letter Ѯ came from Xi.
In mathematics, both uppercase Ξ and lowercase ξ are used to refer to variants of Riemann Xi function (itself a variant of Riemann zeta function). In physics, the uppercase Ξ is used to represent the cascade particle and xi baryon.