in Geometry, a curve generated by a point in the circumference of a circle which rolls along the circumference of another circle, either on the convex or the concave side of its circumference. When generated in the latter way, the curve is sometimes called a hypocycloid. See Conic Sections, part ii. prop. xxv., xxvi., xxvii., xxviii.

If a moveable circle roll along the concave circumference of a fixed circle of twice its diameter, any given point in the circumference of the smaller circle will describe a straight line, which is the diameter of the larger circle. This beautiful property has been applied in mechanics to the production of a rectilineal alternating motion from a continued circular motion. See Mechanics.