epicycle

noun4

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.

   • Is it not [Philosophie], that […] teacheth miserie, famine and sicknesse to laugh? Not by reason of some imaginarie Epicicles, but by naturall and palpable reasons. — 2006
2. •
   Any circle whose circumference rolls around that of another circle, thus creating a hypocycloid or epicycloid.
3. •
   (figurative) An ad hoc complication added to a model to make it fit the known data

   • If two chronicles seemed contradictory, instead of trying to choose between them, a rationalization (epicycle) was devised to cover both. — 1978
4. •
   A ring of atoms joining parts of an already cyclic compound