Astronomy Answers
From the Astronomical Dictionary

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The description of the word you requested from the astronomical dictionary is given below.

the epoch

An epoch is

1. the beginning (the start of day 1 of year 1) of a calendar. This need not be the first day on which the calendar is used. For example, the epoch of the Gregorian calendar lies over 1500 years before the first day on which it was used.
2. the time for which a set of orbital elements is valid. Predictions of positions based on those orbital elements will become less accurate the further from the epoch the time is for which the prediction is made.