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A surface normal or, simply, normal to a flat surface is a three-dimensional vector which is perpendicular to said surface. A normal to a non-flat surface at a point, p, on the surface is a vector which is perpendicular to the tangent plane to that surface at p. The word "normal" is also used as an adjective as well as a noun with this meaning: a line normal to a plane, the normal component of a force, the normal vector, etc. The concept of normality generalizes to orthogonality.