center of symmetry

A center of symmetry of a figure is a point O such that the figure is invariant under a half-turn about O. In other words, for every point M of the figure, its image M' under reflection in O also belongs to the figure. A figure having a center of symmetry is said to be symmetric about that point. When a figure has several axes of symmetry, their point of intersection is a center of symmetry of the figure. For example, the center of a circle is a center of symmetry; the center of a parallelogram is the intersection of its diagonals; and the center of a rectangle or a rhombus is the intersection of its axes of symmetry.