surface normal

Noun

 * 1)  A unit vector at any given point P of a surface M which is perpendicular to the tangent plane TP(M) of M at P. For a surface with parametrization $$ \vec x(u,v) $$, it is given by
 * $$ \vec n = \pm {\vec x_u \times \vec x_v \over \lVert \vec x_u \times \vec x_v \rVert} $$
 * where the sign can be chosen at will.