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Noun: directrix (directrices)  di'rek-triks or dI'rek-triks
  1. 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
  2. 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

Derived forms: directrices

Encyclopedia: Directrix