də̇ˈrektriks, dīˈ- noun
( plural directrixes -ksə̇z ; also directri·ces də̇ˈrektrəˌsēz, dīˈ-; də̇ˌrekˈtrī(ˌ)sēz, (ˌ)dīˌ-)
Etymology: Medieval Latin, feminine of Late Latin director — more at director
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
3. : the center line of the field of fire of an artillery piece