Xi (upper case Ξ, lower case ξ) is the 14th letter of the Greek alphabet. It is pronounced zye by English speakers, but ksee by modern Greeks. In the system of Greek numerals it has a value of 60. The lower-case letter ξ is used as a symbol for:

• Random Variables
• Extent of Reaction (a topic found most often in chemical engineering kinetics)
• Eigenvectors

The upper-case letter Ξ is used as symbol for:

• The 'cascade particles' in particle physics.

Xi was also adopted into the early Cyrillic alphabet, as the letter ksi (, ).

Greek letters

Ξ | Xi (lizherenn) | Кси | Xi (bogstav) | Xi | Ξι | Ξ | Xi | Xi | Xi | Ξ | Xi (lettera) | קסי | Xi | Ξ | Xi | Ksi | Ξ | Xi | Ξ