directrix
noun
di·rec·trix də-ˈrek-triks 
plural directrixes də-ˈrek-trik-səz also directrices də-ˈrek-trə-ˌsēz
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