A real-valued function modelling the length of a vectOR. The nORm must be homogeneous and symmetric and fulfil the following condition: the shORtest way to reach a point is to go straight toward it. Every convex symmetric closed surface surrounding point 0 introduces a nORm by means of Minkowski functional all vectORs that end on the surface have the same nORm then. The most popular nORm is the Euclidean nORm. (2004-02-15)