directrix

n.; pl. E. Directrixes L. Directrices 1. A directress. [ R. ] Jer. Taylor. [ 1913 Webster ]

2. (Geom.) (a) 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) 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. [ 1913 Webster ]