The Collaborative International Dictionary of English v.0.48:

Cissoid \Cis"soid\, n. [Gr. ? like ivy; ? ivy + e'i^dos 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.
   [1913 Webster]