1. planet's circular motion: in the Ptolemaic theory of the solar system, a circle that is followed by a planet, the circle itself being centered on a larger circle within which is the Earth. The epicycle accounts for irregularities of planetary motion in geocentric astronomy.
2. circle that rolls around another: a circle that rolls around the circumference of another circle, either inside or outside
[14th century. Via French or late Latin< Greek epikuklos< kuklos "circle"]
