A lowercase Greek letter commonly used as a variable in physics and mathematics, particularly in quantum mechanics.
Examples
Angular velocity in physics
\upsilon = \frac{d\theta}{dt}
Velocity component in fluid dynamics
\vec{v} = (\upsilon_x, \upsilon_y, \upsilon_z)
Statistical degrees of freedom
\chi^2_{\upsilon} \text{ where } \upsilon = n-1