Definition of Quadratrix:

part of speech: noun

In geom., a curve by means of which right lines equal to the circumferences of circles or other curves, and their various parts, may be found mechanically.

Usage examples:

• About 420 B. C. Hippias of Elis invented a certain curve called the quadratrix by means of which he could square the circle and trisect any angle. David Eugene Smith in "The Teaching of Geometry".