Definitions
from Wiktionary, Creative Commons Attribution/ShareAlike License
 n. 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.
from the GNU version of the Collaborative International Dictionary of English
 n. 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 The Century Dictionary and Cyclopedia
 n. A curve of the third order and third class, having a cusp at the origin and a point of inflection at infinity.
 n. 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.
Etymologies
from Wiktionary, Creative Commons Attribution/ShareAlike License
Ancient Greek, meaning "ivylike".
Examples

Diocles (about the end of the second century B. C.) is known as the discoverer of the _cissoid_ which was used for duplicating the cube.

I was so fascinated by the shape and mathematical description of a simple curve (cardioid or cissoid per  haps) that I had stumbled across in my reading that again I could not rest until I had explored in depth as many curves as I could
fbharjo commented on the word cissoid
How cissoidish! as opposed to the conchoid of Nicomedes
As my father used to say when I asked him what he did during his workday:
"I went around in square circles."
July 13, 2012
ruzuzu commented on the word cissoid
"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."
Websters (1913)
Gotta love a dictionary that uses viz.
July 13, 2012