A line of symmetry is a line such that every point of a figure on one side of the line corresponds to a symmetric point of the figure at the same distance on the other side of the line. That is, the line of symmetry is a perpendicular bisector of the line between the symmetric points.

A line diving the figures into two congruent parts as one is the congruent image of the other.

A line of symmetry is a line drawn between a picture to show a reflection of it. Symmetry lines are only used if the picture can be reflected like a mirror if a thin line is drawn in between of it. For example a perfect square or a perfect circle, when drawn a thin line between the picture, shows a kind of reflection of one side. This is because the area of all sides is equal. The reflection is just an illusion. Other names of line of symmetry are mirror symmetry, bilateral symmetry and reflection symmetry. This line is usually drawn on two dimensional figures. Symmetry are a part of geometry and are used to measure the equality of a figure.