The word epoch can mean either an interval of time, or a particular point in time used as a reference point.
In common usage, the term is often used to apply to a period of time when significant related events took place.
In astronomy, an epoch is a moment in time for which celestial coordinates or orbital elements are specified. In the case of celestial coordinates, the position at other times can be computed by taking into account precession and proper motion. In the case of orbital elements, it is necessary to take account of perturbation by other bodies in order to calculate the orbital elements for a different time.