Directrix

directrix [də′rek·triks]

(mathematics)

A fixed line used in one method of defining a conic; the distance from this line divided by the distance from a fixed point (called the focus) is the same for all points on the conic.

A curve through which a line generating a given ruled surface always passes.

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Directrix 

in artillery, a specially equipped sector of an artillery range or troop training center: targets are installed, artillery fire positions and observation points are rigged out, and so on. These sectors are designed for artillery fire (artillery directrix), tank fire (tank directrix), antiaircraft artillery fire against ground targets (antiaircraft artillery directrix), and other weapons.

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Directrix 

(of a ruled surface), a curve along which a point on a line moves such that the ruled surface is circumscribed by the motion of the line. Any curve intersecting all the generatrices may be taken as the directrix.