surface normal

noun3

1. •
   A unit vector at any given point P of a surface M which is perpendicular to the tangent plane T_P(M) of M at P. For a surface with parametrization ⃑x(u,v), it is given by
2. •
   ⃑n=±⃑x_u×⃑x_v/ lVert ⃑x_u×⃑x_v rVert
3. •
   where the sign can be chosen at will.