hypotrochoid

from The Century Dictionary.

• noun A curve which can be traced by a point rigidly connected with a circle which rolls upon the interior of another circle.

from the GNU version of the Collaborative International Dictionary of English.

• noun (Geom.) A curve, traced by a point in the radius, or radius produced, of a circle which rolls upon the concave side of a fixed circle. See hypocycloid, epicycloid, and trochoid.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun geometry A geometric curve traced by a fixed point on the radius line outside one circle which rotates inside the perimeter of another circle.