di·rec·trix 
(dĭ-rĕktrĭks, dī-)
(dĭ-rĕktrĭks, dī-)n. pl. di·rec·trix·es or di·rec·tri·ces (dī′rĕk-trīsēz)
1. Mathematics A fixed curve traversed by a generatrix in generating a conic section or a cylinder.
2. The median line in the trajectory of fire of an artillery piece.