[UNIX: prob. from Astronomical timekeeping]
n. 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. System time is measured in seconds or ticks 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 word lengths don't increase by then.
See also wall time.