Definition of "mediatrix"

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.

1598, Robert Tofte, “The Third Part of the Moneths Mind of a Melancholy Lover.”, in Alba. The Month's Minde of a Melancholy Lover.‎ (Poetry), published 1880, page 106

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.

1782, Frances Burney, Cecilia, I.i.11

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

2000, Jean H. Gallier, Curves and surfaces in geometric modeling, page 105