[f. EPICYCLE + -OID.] A curve generated by a point in the circumference of a moveable circle, which revolves on that of a fixed circle; in accurate phraseology the term is now limited to the case in which the moveable circle rolls on the exterior of the other (formerly exterior epicycloid); the name hypocycloid being employed for what was formerly called the interior epicycloid.
c. 1790. Imison, Sch. Arts, I. 36.
1816. J. Smith, Panorama Sci. & Art, I. 356. The acting faces of the leaves of the pinion should be parts of an interior epicycloid … and the acting surfaces of the teeth of the wheel should be portions of an exterior epicycloid.
1884. F. J. Britten, Watch & Clockm., 289. The action should be confined as nearly as possible to the epicycloid on the wheel.
