In periodization the term Epoch most often means "a beginning", and epochs thus define a point or moment when what comes after has begun, though some disciplines (geology) use the term as a time division like age, period, or era, meaning a defined span of time. Depending on context, epoch can refer to:
a defining moment in the beginning of, or characteristic of, a distinctive historical period or era.
epoch (astronomy) - a precise moment in time for which celestial coordinates or orbital elements are specified by international agreement, usually periodically reset to centennial and half-century dates. All other orbits calculations near or after the astronomical epoch are based on the epoch date.
Many computer operating systems maintain system time based on some starting time called the epoch. For example, Unix and POSIX systems count time as seconds since January 11970. See Unix time.
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