directrix

directrix is a Noun

Definition:

[1] A directress. [R.] Jer. Taylor.

[2] 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.

[3] 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.