directrix
di.rec.trix \-triks\ n [ML, fem. of LL director, fr. L directus, pp.] archaic 1: DIRECTRESS 2a: a fixed line by relation to which a conic section is described 2b: a line or curve with which a generatrix of a surface remains in contact
Webster's English Dictionarydi.rec.trix \-triks\ n [ML, fem. of LL director, fr. L directus, pp.] archaic 1: DIRECTRESS 2a: a fixed line by relation to which a conic section is described 2b: a line or curve with which a generatrix of a surface remains in contact