Epicycle

Part of speech: noun (n.)

1. 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. The schoolmen were like astronomers which did feign eccentries, and epicycles, and such engines of orbs. Bacon.

2. A circle which rolls on the circumference of another circle, either externally or internally.