The Gaussian curvature of a surface at a point $p$ is the product $$ K = \kappa_1 \kappa_2 $$ of the two principal curvatures of the surface at $p$

The arithmetic mean of the principal curvatures at a point $p$ $$ H = \frac{\kappa_1 + \kappa_2}{2} $$ is called the mean curvature of the surface at $p$