mediatrix

noun

1. A female mediator.

   “My lifes Cataſtrophe is at an end, / The Staffe whereon my ſickly Loue did leane / And which from falling (ſtill) did him defend, / Is through miſchance in ſunder broken cleane. / Gone is my Mediatrix, my beſt Aduocate, / Who vſde for me to interceſsionate.”

   “He promised, however, to speak to Mr. Harrel upon the subject, but the promise was evidently given to oblige the fair mediatrix, without any hope of advantage to the cause.”

2. (rare)A bisector, the line that is perpendicular to a line segment and intersects the line segment at its midpoint.

   “[…] the intersection of the normal at M to the parabola with the mediatrix of the line […]”