Epoch

The "epoch" serves as a reference point from which time is measured.

In a more gradual change, a deciding moment is chosen when the epoch criterion was reached.

An epoch in astronomy is a reference time used for consistency in calculation of positions and orbits.

A common astronomical epoch is J2000, which is noon on January 1, 2000, Terrestrial Time.

An epoch in Geochronology is a time period, typically in the order of tens of millions of years.