epoch

1. a particular period of time marked by distinctive features, events, etc.: The treaty ushered in an epoch of peace and good will. 2. the beginning of a distinctive period in the history of anything: The splitting of the atom marked an epoch in scientific discovery. 3. a point of time distinguished by a particular event or state of affairs; a memorable date: His coming of age was an epoch in his life. 4. Geology. any of several divisions of a geologic period during which a geologic series is formed. Compare age (def 12). 5. 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. the mean longitude of a planet as seen from the sun at such an instant or date. 6. Physics. the displacement from zero at zero time of a body undergoing simple harmonic motion.