Webster's Revised Unabridged Dictionary · 1913

Epoch

Epoch (ep"ok or ē"pok; 277) , noun

[Late Latin epocha, Greek 'epochh` check, stop, an epoch of a star, an historical epoch, from 'epe`chein to hold on, check; 'epi` upon + 'e`chein to have, hold; akin to Sanskrit sah to overpower, Gothic sigis victory, Anglo-Saxon sigor, sige, German sieg: compare French époque. See Scheme.]

1.
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.
In divers ages,... divers epochs of time were used. — Usher
Great epochs and crises in the kingdom of God. — Trench
The acquittal of the bishops was not the only event which makes the 30th of June, 1688, a great epoch in history. — Macaulay

Epochs mark the beginning of new historical periods, and dates are often numbered from them.

2.
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.
So vast an epoch of time. — F. Harrison
The influence of Chaucer continued to live even during the dreary interval which separates from one another two important epochs of our literary history. — A. W. Ward
3.
(Geology) A division of time characterized by the prevalence of similar conditions of the earth; commonly a minor division or part of a period.
The long geological epoch which stored up the vast coal measures. — J. C. Shairp
4.
(a) (Astronomy) The date at which a planet or comet has a longitude or position.
(b)
(Astronomy) 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.
The capture of Constantinople is an epoch in the history of Mahometanism; but the flight of Mahomet is its era. — C. J. Smith