Directrix
Directrix , noun
1.
A directress. [Rare] — Jer. Taylor
2.
(a) (Geometry) A line along which a point in another line moves, or which in any way governs the motion of the point and determines the position of the curve generated by it; the line along which the generatrix moves in generating a surface.
(b)
(Geometry) A straight line so situated with respect to a conic section that the distance of any point of the curve from it has a constant ratio to the distance of the same point from the focus.