- a line passing through the center of the surface of a lens or spherical mirror and through the centers of curvature of all segments of the lens or mirror.
- one of three mutually perpendicular axes of a body about which the moment of inertia is maximum or minimum.
- the line passing through the optical centre and centres of curvature of the faces of a lens or a curved mirror
- any of three mutually perpendicular axes about which the moment of inertia of a body is maximum
