**Ep`i*cy"cloid** (?), n. [*Epicycle* + *-oid*: cf. F. *'epicycloide*.] Geom.

A curve traced by a point in the circumference of a circle which rolls on the convex side of a fixed circle.

⇒ 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.

© Webster 1913.