- a small circle the center of which moves around in the circumference of a larger circle: used in Ptolemaic astronomy to account for observed periodic irregularities in planetary motions.
- a circle that rolls, externally or internally, without slipping, on another circle, generating an epicycloid or hypocycloid.
- (in the Ptolemaic system) a small circle, around which a planet was thought to revolve, whose centre describes a larger circle (the deferent) centred on the earth
- a circle that rolls around the inside or outside of another circle, so generating an epicycloid or hypocycloid
