epicycle
    n 1: a circle that rolls around (inside or outside) another
         circle; generates an epicycloid or hypocycloid

Epicycle \Ep"i*cy`cle\, n. [L. epicyclus, Gr. ?; 'epi` upon + ?
   circle. See Cycle.]
   1. (Ptolemaic Astron.) A circle, whose center moves round in
      the circumference of a greater circle; or a small circle,
      whose center, being fixed in the deferent of a planet, is
      carried along with the deferent, and yet, by its own
      peculiar motion, carries the body of the planet fastened
      to it round its proper center.
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            The schoolmen were like astronomers which did feign
            eccentrics, and epicycles, and such engines of orbs.
                                                  --Bacon.
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   2. (Mech.) A circle which rolls on the circumference of
      another circle, either externally or internally.
      [1913 Webster] epicyclic