epicycle

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun In Ptolemaic cosmology, a small circle, the center of which moves on the circumference of a larger circle at whose center is the earth and the circumference of which describes the orbit of one of the planets around the earth.
• noun Mathematics A circle whose circumference rolls along the circumference of a fixed circle, thereby generating an epicycloid or a hypocycloid.

from The Century Dictionary.

• noun A circle moving upon or around another circle, as one of a number of wheels revolving round a common axis. See epicyclic train, under epicyclic.
• noun In the Ptolemaic system of astronomy, a little circle, conceived for the explanation of planetary motion, whose center was supposed to move round in the circumference of a greater circle; a small circle whose center, being fixed in the deferent of a planet, was supposed to be carried along with the deferent, and yet by its own peculiar motion to carry the body of the planet fastened to it round its proper center. Copernicus also made use of epicycles, which, however, were banished by Kepler.
• noun In mod. astron., sometimes used for the geocentric path of a planet, or its path relative to the earth regarded as fixed.

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

• noun (Ptolemaic Astron.) A circle, whose center moves round in the circumference of a greater circle; or a small circle, whose center, being fixed in the deferent of a planet, is carried along with the deferent, and yet, by its own peculiar motion, carries the body of the planet fastened to it round its proper center.
• noun (Mech.) A circle which rolls on the circumference of another circle, either externally or internally.

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

• noun astronomy A small circle whose centre is on the circumference of a larger circle; in Ptolemaic astronomy it was seen as the basis of revolution of the "seven planets", given a fixed central Earth.
• noun mathematics Any circle whose circumference rolls around that of another circle, thus creating a hypocycloid or epicycloid.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun a circle that rolls around (inside or outside) another circle; generates an epicycloid or hypocycloid