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
geometriccurve traced by a fixed point on the radius line outside one circle which rotates inside the perimeter of another circle.
from Wiktionary, Creative Commons Attribution/Share-Alike License
hypo- + trochoid
I saw a hypotrochoid drawing kit for sale around Christmas this year, and not the canonical Spirograph, either.