Informally, it is just an unspecified short period of time. However, different disciplines over time have defined specific time intervals for the term.
In electronics, the term means 1/60 or 1/50 of a second.
In computing, it is not an exact measure of time, but it is used to describe one tick of the system timer interrupt. Alternatively, it could be 1/250 of a second.
In physics, a jiffy is the time taken for light to travel to a specified distance, or the time it takes for light to travel one fermi making a jiffy 3 x 10 (to the -29th power)seconds.
The term "jiffy" is sometimes used in computer animation as a method of defining playback rate, with the delay interval between individual frames specified in 1/100th-of-a-second (10 ms) jiffies.