directrix

noun

di·​rec·​trix də-ˈrek-triks 

plural directrixes də-ˈrek-trik-səz  also directrices də-ˈrek-trə-ˌsēz 

1

archaic : directress

2

: a fixed curve with which a generatrix maintains a given relationship in generating a geometric figure

specifically : a straight line the distance to which from any point of a conic section is in fixed ratio to the distance from the same point to a focus

Word History

Etymology

Medieval Latin, feminine of Late Latin director, from Latin dirigere

First Known Use

1622, in the meaning defined at sense 1