The Collaborative International Dictionary of English v.0.48:

Conchoid \Con"choid\, n. [Gr. ?; ? shell + e'i^dos form: cf. F.
   concho["i]de.] (Geom.)
   A curve, of the fourth degree, first made use of by the Greek
   geometer, Nicomedes, who invented it for the purpose of
   trisecting an angle and duplicating the cube.
   [1913 Webster]