A “jiffy” actually has a formal definition. More than one, in fact.
In electronics, a jiffy is the time between alternating current power cycles, 1/60 or 1/50 of a second in most countries.
In astrophysics and quantum physics a jiffy is the time it takes for light to travel one fermi, which is the size of a nucleon.
In computing, a jiffy is the duration of one tick of the system timer interrupt. It is not an absolute time interval unit, since its duration depends on the clock interrupt frequency of the particular hardware platform.