Informally, a “jiffy” simply means “a moment” or “quickly.” (Think “I’ll be there in a jiffy!”) But a jiffy can be a precise measurement of time, too, although its definition varies between different fields of study.
What are jiffies??
The global variable jiffies holds the number of ticks that have occurred since the system booted. On boot, the kernel initializes the variable to zero, and it is incremented by one during each timer interrupt. Thus, because there are HZ timer interrupts in a second, there are HZ jiffies in a second.
Beginnings in measurement
The earliest technical usage for jiffy was defined by Gilbert Newton Lewis (1875–1946). He proposed a unit of time called the “jiffy” which was equal to the time it takes light to travel one centimeter in a vacuum (approximately 33.3564 picoseconds). It has since been redefined for different measurements depending on the field of study.
In astrophysics and quantum physics, a jiffy is the amount of time it takes for light to travel a distance the size of a nucleus. But the absolute smallest definite measurement of time may be the Planck time. The Planck time shares a name with the Planck length, which is the smallest possible length at 1.61619926 × 10^-35 meters. Knowing the Planck length is necessary in determining the smallest possible amount of time. The Planck time is the smallest amount of time, and it can be calculated by dividing the Planck length by the speed of light. The Planck time is something around 10^-43 seconds.