The large circular orbit around which a planet was thought to orbit, in one or many epicycles: A deferentin the time of the Ptolemaic astronomy, was an imaginary circle that surrounded the planet Earth, around which the heavenly bodies orbited.

An "epicycle" is a circular orbit of a body around a point that is itself in a circular orbit round a parent body.

Such a system was formulated to explain some planetary orbits in the solar system before they were known to be elliptical.