A figure in the plane has line symmetry if the figure can be mapped onto itself by a reflection in a line. This line of reflection is called the line of symmetry. When a figure has line symmetry, it is said to be reflectionally symmetric or line symmetric.

Some figures can have more than one line of symmetry. For example, a square has four — the lines passing thought the midpoints of opposite sides and the lines containing the diagonals of the square.