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 $\ell$ maps every point $A$ in the plane onto its image $A^{\prime }$ such that one of the following statements is satisfied.

• If $A$ is on the line $\ell ,$ then $A$ and $A^{\prime }$ are the same point.
• If $A$ is not on the line $\ell ,$ then $\ell$ is the perpendicular bisector of $\overline{A{A}^{\prime }}.$

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.