n. 本轮

[ noun ]
a circle that rolls around (inside or outside) another circle; generates an epicycloid or hypocycloid
<noun.shape>

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.

The schoolmen were like astronomers which did feign
eccentrics, and epicycles, and such engines of orbs.
--Bacon.

2. (Mech.) A circle which rolls on the circumference of
another circle, either externally or internally.