A period or series of events or numbers which recur everlastingly in precisely the same order.
Cycle of the moon, called “Meton's Cycle,” from Meton, who discovered it, is a period of nineteen years, at the expiration of which time the phases of the moon repeat themselves on the same days as they did nineteen years previously. (See Callipic Period.)
Cycle of the sun. A period of twenty-eight years, at the expiration of which time the Sunday letters recur and proceed in the same order as they did twenty-eight years previously. In other words, the days of the month fall again on the same days of the week.
The Platonic cycle or great year is that space of time which elapses before all the stars and constellations return to any given state. Tycho Brahët calculated this period at 25,816 years, and Riccioli at 25,920.
Source: Dictionary of Phrase and Fable, E. Cobham Brewer, 1894