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. [ 1913 Webster ]

☞ 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. [ 1913 Webster ]

a. Pertaining to the epicycloid, or having its properties. [ 1913 Webster ]

Epicycloidal wheel, a device for producing straight-line motion from circular motion, on the principle that a pin fastened in the periphery of a gear wheel will describe a straight line when the wheel rolls around inside a fixed internal gear of twice its diameter. [ 1913 Webster ]