A lowercase Greek letter commonly used to represent unknown variables or parameters in mathematical analysis and statistics.
Examples
Random variable notation in probability theory
P(\xi > x) = \int_x^\infty f_\xi(t)\,dt
Coordinate variable in parametric equations
x = \xi\cos(t), \quad y = \xi\sin(t)
Eigenvalue problem in differential equations
-\frac{d^2\xi}{dx^2} = \lambda\xi