n. A directress.\nn. A line along which a point in another line moves, or\n which in any way governs the motion of the point and determines the\n position of the curve generated by it; the line along which the\n generatrix moves in generating a surface.\nn. A straight line so situated with respect to a conic\n section that the distance of any point of the curve from it has a\n constant ratio to the distance of the same point from the focus.