Definition of Platonic year in US English:

Platonic year

noun

historical
  • (Originally) a cycle postulated by some ancient astronomers, in which the celestial objects go through all their possible movements and return to their original relative positions, after which (according to some versions of the theory) all history repeats itself; (in later use identified with) the period of precession of the equinoxes (approx. 25,800 years).

Origin

Mid 17th century; earliest use found in Thomas Fuller (1607/8–1661), Church of England clergyman.