A reflection is a transformation in which every point of a figure is reflected across a line. The line across the points are reflected in what is called the line of reflection. This acts like a mirror. In more precise terms, a reflection across a line l maps every point A in the plane onto its image A' such that one of the following statements is satisfied.

• If A is on the line l, then A and A' are the same point.
• If A is not on the line l, then l is the perpendicular bisector of AA'.

Like rotations and translations, reflections are rigid motions because they preserve the side lengths and angle measures. However, reflections can change the orientation of the preimage.