1. The duration of one tick of the computer's system clock. Often one AC cycle time (1/60 second in the US and Canada, 1/50 most other places), but more recently 1/100 sec has become common.
2. Confusingly, the term is sometimes also used for a 1-millisecond wall time interval. 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*.