cissoid

from The Century Dictionary.

• noun A curve of the third order and third class, having a cusp at the origin and a point of inflection at infinity.
• noun It was invented by one Diocles, a geometer of the second century b. c., with a view to the solution of the famous problem of the duplication of the cube, or the insertion of two mean proportionals between two given straight lines. Its equation is x=y (a—x). In the cissoid of Diocles the generating curve is a circle; a point A is assumed on this circle, and a tangent M M' through the opposite extremity of the diameter drawn from A; then the property of the curve is that if from A any oblique line be drawn to M M', the segment of this line between the circle and its tangent is equal to the segment between A and the cissoid. But the name has sometimes been given in later times to all curves described in a similar manner, where the generating curve is not a circle.
• Included between the concave sides of two intersecting curves: as, a cissoid angle.

from the GNU version of the Collaborative International Dictionary of English.

• noun (Geom.) A curve invented by Diocles, for the purpose of solving two celebrated problems of the higher geometry; viz., to trisect a plane angle, and to construct two geometrical means between two given straight lines.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun geometry Any of a family of curves defined as the locus of a point, P, on a line from a given fixed point and intersecting two given curves, C1 and C2, where the distance along the line from C1 to P remains constant and equat to the distance from P to C2.