A lowercase Greek letter commonly used to denote curvature in differential geometry and thermal conductivity in physics.
Examples
Heat conductivity in physics
q = -\kappa \frac{dT}{dx}
Cohen-Macaulay ring condition
\kappa(R) = \dim R - \text{depth } R = 0
Curvature of a curve
\kappa(t) = \frac{|x'(t)y''(t) - y'(t)x''(t)|}{(x'(t)^2 + y'(t)^2)^{3/2}}