Geom. the curve traced by a point on the circumference of an epicycle that rolls around the outside of a fixed circle
Origin of epicycloidepicycl(e) + -oid
The curve described by a point on the circumference of a circle as the circle rolls on the outside of the circumference of a second, fixed circle.
The parametric equations of an epicycloid are
x = (a + b) cos &thgr; - b cos (a + bb
y = (a + b) sin &thgr; - b sin (a + bb