1. 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.
The schoolmen were like astronomers which did feign eccentrics, and epicycles, and such engines of orbs. --Bacon.
2. Mech. A circle which rolls on the circumference of another circle, either externally or internally.
n : a circle that rolls around (inside or outside) another
circle; generates an epicycloid or hypocycloid