The Collaborative International Dictionary of English v.0.48:

Directrix \Di*rect"rix\, 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]