Geom. the curve traced by a point on the circumference of an epicycle that rolls around the inside of a fixed circle
See hypocycloid in American Heritage Dictionary 4
(hīˌpō-sīˈkloidˌ)
noun
The plane locus of a point fixed on a circle that rolls on the inside circumference of a fixed circle.
(click for a larger image)
hypocycloid
If a equals the radius of a fixed circle and b equals the radius of a smaller rotating circle, the parametric equations of the hypocycloid are: x = (a - b) cosΘ + b cos [(a - b)Θ]/b