Xi (upper case Ξ, lower case ξ) is the 14th letter of the Greek alphabet. 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 in chemical engineering)

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

• The 'cascade particles' in particle physics.