\Ep`i*cy"cloid\, n. [Epicycle + -oid: cf. F.
['e]picyclo["i]de.] (Geom.)
A curve traced by a point in the circumference of a circle
which rolls on the convex side of a fixed circle.
Note: Any point rigidly connected with the rolling circle,
      but not in its circumference, traces a curve called an
      epitrochoid. The curve traced by a point in the
      circumference of the rolling circle when it rolls on
      the concave side of a fixed circle is called a
      hypocycloid; the curve traced by a point rigidly
      connected with the rolling circle in this case, but not
      its circumference, is called a hypotrochoid. All the
      curves mentioned above belong to the class class called
      roulettes or trochoids. See {Trochoid}.