1. The duration of one tick of the system clock on your computer (see tick). Often one AC cycle time (1/60 second in the U.S. and Canada, 1/50 most other places), but more recently 1/100 sec has become common. “The swapper runs every 6 jiffies” means that the virtual memory management routine is executed once for every 6 ticks of the clock, or about ten times a second.
2. Confusingly, the term is sometimes also used for a 1-millisecond wall time interval.
3. Even more confusingly, physicists semi-jokingly use ‘jiffy’ to mean the time required for light to travel one foot in a vacuum, which turns out to be close to one nanosecond. Other physicists use the term for the quantum-nechanical lower bound on meaningful time lengths,
4. Indeterminate time from a few seconds to forever. “I'll do it in a jiffy” means certainly not now and possibly never. This is a bit contrary to the more widespread use of the word. Oppose nano. See also Real Soon Now.