An Etymological Dictionary of Astronomy and Astrophysics

1. A sequence of changes that are repeated regularly, such as revolution, rotation, vibration, oscillation, wave motion.
   

2. One complete set of changes in the value of a → periodic function during one period.
   

3. A period of time at the end of which an event or sequence of events are repeated.
   → carbon cycle, → Carnot cycle, → CNO cycle, → cycle shape, → hydrologic cycle, → Metonic cycle, → Milankovich cycle, → solar activity cycle, → solar cycle, → solar magnetic cycle, → sunspot cycle, → epicycle, → recycle.
   

4. In → graph theory, a closed → path with at least one → edge.

Etymology (EN): From L.L. cyclus, from Gk. kyklos “circle, wheel,” from PIE base *kw(e)-kwl-o- “wheel, circle,” from *kwel- “to turn, move around, sojourn, dwell,” (cf. Av. caxra- “wheel,” caraiti “he moves, approaches;”
Mod.Pers. carx “wheel;” Skt. cakra- “wheel, circle; cycle,” carati “he moves, wanders;” Gk. polos “axis of a sphere,” polein “move around;” L. colere “to dwell in, to cultivate, move around,” colonus “farmer, settler;” O.E. hweol “wheel;” Rus. koleso “awheel”).

Etymology (PE): Carx “every thing performing a circulatory motion; a wheel; a cart,” Mid.Pers. chr “wheel,” Parthian cxr “wheel,” Ossetic, Khotanese calx “wheel,” Av. caxra- “wheel,” cognate with Gk. kyklos “circle, wheel,” as above. Carxé from carx + nuance suffix -é.