noun
1.
a. : any of three mutually perpendicular axes through a given point of a rigid body with respect to which the moment of inertia is either a maximum or a minimum — compare permanent axis
b. : the line with respect to which a spherical mirror or lens system is symmetrical and which passes through both the center of the surfaces and their centers of curvature
2. : the line through a focus of a conic perpendicular to a directrix