epicycle

Meanings

1. A small circle whose centre is on the circumference of a larger circle; in Ptolemaic astronomy it was seen as the basis of revolution of the "seven planets", given a fixed central Earth.
2. Any circle whose circumference rolls around that of another circle, thus creating a hypocycloid or epicycloid.
3. A ring of atoms joining parts of an already cyclic compound
4. An ad hoc complication added to a model to make it fit the known data

Etymology

From Latin epicyclus, from Ancient Greek ἐπίκυκλος (epíkuklos), from ἐπί (epí, “upon”) + κύκλος (kúklos, “circle”). The ad hoc complication sense is a generalization from the archetype supplied by the Ptolemaic astronomy sense.