Cycloid (s?kloid), n. [Cyclo- + -oid: cf. F. cycloïde.] (Geom.) A curve generated by a point in the plane of a circle when the circle is rolled along a straight line, keeping always in the same plane.
[1913 Webster]

&hand_; The common cycloid is the curve described when the generating point (p) is on the circumference of the generating circle; the curtate cycloid, when that point lies without the circumference; the prolate or inflected cycloid, when the generating point (p) lies within that circumference.
[1913 Webster]

Cycloid, n. (Zoöl.) One of the Cycloidei.
[1913 Webster]