hypocycloid in American English
the curve traced by a point on the circumference of an epicycle that rolls around the inside of a fixed circle
noun: a curve generated by the motion of a point on the circumference of a circle that rolls internally, without slipping, on a given circle
hypocycloid in British English
noun: a curve described by a point on the circumference of a circle as the circle rolls around the inside of a fixed coplanar circle