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Greek alphabet
Α α Alpha	Ν ν Nu
Β β Beta	Ξ ξ Xi
Γ γ Gamma	Ο ο Omicron
Δ δ Delta	Π π Pi
Ε ε Epsilon	Ρ ρ Rho
Ζ ζ Zeta	Σ σ ς Sigma
Η η Eta	Τ τ Tau
Θ θ Theta	Υ υ Upsilon
Ι ι Iota	Φ φ Phi
Κ κ Kappa	Χ χ Chi
Λ λ Lambda	Ψ ψ Psi
Μ μ Mu	Ω ω Omega
Obsolete letters
Ϝ ϝ Digamma	Ϻ ϻ San
Ϙ ϙ Qoppa	Ϡ ϡ Sampi
Ϛ ϛ Stigma	Ϸ ϸ Sho

Xi (upper case Ξ, lower case ξ) is the 14th letter of the Greek alphabet. It is pronounced zai by English speakers, but ksi by modern Greeks. The modern Greek pronunciation has also become fairly common among mathematicians, even speaking in English. 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
• A parameter in a Generalized Pareto Distribution

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

• The 'cascade particles' in particle physics.
• indicating "no change of state" in Z notation.

Xi was also adopted into the early Cyrillic alphabet, as the letter ksi (Ѯ, ѯ).als:Ξ br:Xi (lizherenn) bg:Кси ca:Xi da:Xi (bogstav) de:Xi arc:Ξ el:Ξι es:Ξ eu:Xi fr:Xi gd:Xi gl:Xi ko:Ξ it:Xi (lettera) he:קסי hu:Xi nl:Xi (letter) ja:Ξ nds:Xi pl:Ksi pt:Ξ ru:Кси (буква) sv:Xi zh:Ξ