normal

A polygon and its two normal vectors A normal to a surface at a point is the same as a normal to the tangent plane to the surface at the same point. Tangent and normal to a curve in a Cartesian coordinate system.

In geometry, a normal is an object (e.g. a line, ray, or vector) that is perpendicular to a given object. For example, the normal line to a plane curve at a given point is the infinite straight line perpendicular to the tangent line to the curve at the point.