A point p on a regular surface M element R^3 is said to be planar if the Gaussian curvature K(p) = 0 and S(p) = 0 (where S is the shape operator), or equivalently, both of the principal curvatures κ_1 and κ_2 are 0.

anticlastic | elliptic point | Gaussian curvature | hyperbolic point | parabolic point | synclastic