A fixed point of time, established in history by the
occurrence of some grand or remarkable event; a point of time marked by
an event of great subsequent influence; as, the epoch of the creation;
the birth of Christ was the epoch which gave rise to the Christian era.

A period of time, longer or shorter, remarkable for events
of great subsequent influence; a memorable period; as, the epoch of
maritime discovery, or of the Reformation.

A division of time characterized by the prevalence of
similar conditions of the earth; commonly a minor division or part of a
period.

The date at which a planet or comet has a longitude or
position.

An arbitrary fixed date, for which the elements used in
computing the place of a planet, or other heavenly body, at any other
date, are given; as, the epoch of Mars; lunar elements for the epoch
March 1st, 1860.