center of symmetry
or center, n. a point around which a curve is symmetric, so that for every point P on the curve there is another P' such that the directed lines joining the center to P and to P' are of equal length and opposite sense. For example the sine curve

has a center of symmetry about the origin, O, with respect to which P and P' are images of one another, but it has no axis of symmetry. Compare axis of symmetry.