The beginning of a distinctive period in the history of anything.
A particular period of time marked by distinctive features or events.
A memorable date.
Geology. Any of several divisions of a geologic period during which a geologic series is formed. Compare age.
Astronomy. An arbitrarily fixed instant of time or date, usually the beginning of a century or half century, used as a reference in giving the elements of a planetary orbit or the like. b.The mean longitude of a planet as seen from the sun at such an instant or date.
Physics. The displacement from zero at zero time of a body undergoing simple harmonic motion.
As a result, lawmakers can now submit draft constitutional amendments to the Diet, a practice that had been frozen for three years since the national voting bill was enacted. This is an epoch-making event in the postwar history of our nation's constitutional politics.
-- "Lawmakers must face constitutional change", editorial, The Daily Yomiuri, May, 2010.
Indeed, I've seen this epoch as an increasingly intimate collaboration between our biological heritage and a future that transcends biology.
-- Ray Kurzweil, The singularity is near: when humans transcend biology
Epoch has acquired a variety of precise meanings through the centuries: historical, in ancient Rome and Greece; geological and astronomical in modern times.