Source : Webster's Revised Unabridged Dictionary (1913)

Cycloid \Cy"cloid\ (s?"kloid), n. [Cyclo- + -oid: cf. F.
   cyclo["i]de.] (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.

   Note: 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.

Cycloid \Cy"cloid\, a. (Zo["o]l.)
   Of or pertaining to the Cycloidei.

   {Cycloid scale} (Zo["o]l.), a fish scale which is thin and
      shows concentric lines of growth, without serrations on
      the margin.

Cycloid \Cy"cloid\, n. (Zo["o]l.)
   One of the Cycloidei.

Source : WordNet®

cycloid
     adj : resembling a circle [syn: {cycloidal}]

cycloid
     n : a line generated by a point on a circle rolling along a
         straight line