In a reflection of a figure in relation to a line (with the name Axis of Reflection or Axis of Symmetry), an image is transformed into another equal figure, in which all its points are at the same distance from the axis of reflection as the original points. The segment formed by the original point and the transformed point is always perpendicular to the axis of symmetry.
All plane isometric transformations can be expressed as compositions of a maximum of three mirror reflections. This fact allows the complete classification of plane isometries. Besides identical transformation, isometric transformations also include mirror reflection, translation, rotation around the center, and glide deflection.