Ep`i*cy"cloid (?), n. [Epicycle + -oid: cf. F. épicycloïde.] (Geom.) A curve traced by a point in the circumference of a circle which rolls on the convex side of a fixed circle.

&fist; 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.

• (geometry) The locus of a point on a circle that rolls along the outside of another circle.