epoch
     n 1: a period marked by distinctive character or reckoned from a
          fixed point or event [syn: {era}]
     2: (astronomy) the precise date that is the point of reference
        for which information (as coordinates of a celestial body)
        is referred [syn: {date of reference}]
     3: a unit of geological time
Source: WordNet® 2.0

epoch n. [Unix: prob. from astronomical timekeeping] The time and date
   corresponding to 0 in an operating system's clock and timestamp values.
   Under most Unix versions the epoch is 00:00:00 GMT, January 1, 1970;
   under VMS, it's 00:00:00 of November 17, 1858 (base date of the U.S.
   Naval Observatory's ephemerides); on a Macintosh, it's the midnight
   beginning January 1 1904. System time is measured in seconds or {tick}s
   past the epoch. Weird problems may ensue when the clock wraps around
   (see {wrap around}), which is not necessarily a rare event; on systems
   counting 10 ticks per second, a signed 32-bit count of ticks is good
   only for 6.8 years. The 1-tick-per-second clock of Unix is good only
   until January 18, 2038, assuming at least some software continues to
   consider it signed and that word lengths don't increase by then. See
   also {wall time}. Microsoft Windows, on the other hand, has an epoch
   problem every 49.7 days - but this is seldom noticed as Windows is
   almost incapable of staying up continuously for that long.


Source: The Jargon File