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In geometry, a normal is an object such as a line, ray, or vector that is
perpendicular to a given object.

For example, in two dimensions, the normal line to a curve at a given point
is the line perpendicular to the tangent line to the curve at the point.

A normal vector may have length one (a unit vector) or
its length may represent the curvature of the object (a curvature vector);
its algebraic sign may indicate sides (interior or exterior).