Xi (uppercase Ξ, lowercase ξ) is the 14th letter of the Greek alphabet. It is pronounced [ksi] in Modern Greek, and generally /zaɪ/ or /saɪ/ in English. In the system of Greek numerals, it has a value of 60. Xi was derived from the Phoenician letter samekh .
Xi is not to be confused with the letter chi, which gave its form to the Latin letter X.
Both in classical Ancient Greek and in Modern Greek, the letter Ξ represents the sound [ks]. In some archaic local variants of the Greek alphabet, this letter was missing. Instead, especially in the dialects of most of the Greek mainland and Euboea, the sound [ks] was represented by Χ (which in classical Greek is chi, used for /kʰ/). Because this variant of the Greek alphabet was used in Italy, the Latin alphabet borrowed Χ rather than Ξ as the Latin letter X.
While having no Latin derivative, the Xi was adopted into the early Cyrillic alphabet, as the letter ksi (Ѯ, ѯ).
The upper-case letter Ξ is used as symbol for:
The lower-case letter ξ is used as a symbol for: