Cissoid

It consists of all those points Q in the plane such that the line through P and Q meets the two curves in two points whose distance apart equals the distance between P and Q.

For example, see the cissoid of Diocles.

Cis"soid (?), n. [Gr. like ivy; ivy + form.] 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.

© Webster 1913.