directrix

di·rec·trix Audio Help (dĭ-rěk'trĭks, dī-)

n. pl. di·rec·trix·es or di·rec·tri·ces (dī'rěk-trī'sēz)

Mathematics A fixed curve traversed by a generatrix in generating a conic section or a cylinder.
The median line in the trajectory of fire of an artillery piece.

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The American Heritage® Dictionary of the English Language, Fourth Edition
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