Xi
From Textus Receptus
Xi (uppercase Ξ, lowercase ξ) is the 14th letter of the Greek alphabet. It is pronounced ˈksi in Modern Greek, and generally ˈzaɪ or ˈsaɪ in English.^{[1]} In the system of Greek numerals, it has a value of 60.
Xi is not to be confused with the letter chi, which gave its form to the Latin letter X.
Contents 
Greek
Both in Modern Greek and in the modern system for writing Ancient Greek, xi represents /ks/. This usage comes from the regional variants of the Greek alphabet used to write the Attic and Ionic dialects.
In the Western or Euboean alphabet, the sounds represented by xi and chi were the opposite of the modern usage: xi represented /kʰ/, and chi (Χ) represented /ks/. Because this variant of the Greek alphabet was used in Italy, the Latin alphabet borrowed chi rather than xi as the Latin letter X.
Cyrillic
While having no Latin derivative, the Xi was adopted into the early Cyrillic alphabet, as the letter ksi (Ѯ, ѯ).
Math and science
Upper case
The uppercase letter Ξ is used as symbol for:
 The "cascade particles" in particle physics
 Indicating "no change of state" in Z notation
 The partition function under the grand canonical ensemble in statistical mechanics
Lower case
The lowercase letter ξ is used as a symbol for:
 Random variables
 Dimensionless distance variable used in the LaneEmden equation
 Extent of reaction (a topic found most often in chemical engineering kinetics)
 Eigenvectors
 Damping Ratio C/Ccr(vibrational analysis)
 A parameter in a generalized Pareto distribution
 The symmetric function equation of the Riemann zeta function in mathematics, also known as the Riemann Xi function
 A universal set in the set theory
 A number used in the error term of Newton–Cotes formulas that falls between a and b
 One of the two different polypeptide chains of the human embryonic hemoglobin types HbPortland (ξ_{2}γ_{2}) and HbGower I (ξ_{2}ε_{2})
 The correlation function in astronomy
 Frequency
 A small displacement in MHD plasma stability theory
 A parameter denoted as warped time used to derive the equations for homogeneous azeotropic distillation
 The xcoordinate of computational space as used in Computational Fluid Dynamics
 Potential difference in physics (in volts)
 The radial integral in the spinorbit matrix operator in atomic physics.
 The Killing vector in General Relativity.
 Average Logarithmic Energy Decrement per Collision (neutron calculations in nuclear physics)
References
Majuscule form  
Α  Β  Γ  Δ  Ε  Ζ  Η  Θ  Ι  Κ  Λ  Μ  Ν  Ξ  Ο  Π  Ρ  Σ  Τ  Υ  Φ  Χ  Ψ  Ω  
Minuscule form  
α  β  γ  δ  ε  ζ  η  θ  ι  κ  λ  μ  ν  ξ  ο  π  ρ  σ  τ  υ  φ  χ  ψ  ω 

