Definition of: cycloid

(sī′kloid) adjective
1. Resembling a circle or somewhat circular: specifically said of fish scales with concentric rings and smooth edges.
2. Psychiatry Of or pertaining to a personality type exhibiting marked alternations of mood.
—noun Geom. The curve described by a point on the circumference of a circle that rolls, without slipping, along a straight line in a single plane. [<Gk. kykloeidēs circular]
—cy·cloi′dal adjective
—cy′cloid·ism noun
CYCLOID
As the circle c rolls along the straight line bd, point a on its circumference traces the cycloid bad.