Transformation of the plane characterized by a line *d* called the line of symmetry and a line *e*, that defines a direction of symmetry. Under this transformation, every point *P* on the plane is mapped to a point *P’* such that the segment *PP’* is parallel to line *e* and its midpoint *M* lies on the axis *d*.

A reflection is an axial symmetry in which the direction e is perpendicular to the line of symmetry.