A Jiffy Is an Actual Unit of Time — And Its Length Depends on Who's Using It

Quick Answer

A "jiffy" is a genuine unit of time, though its exact duration depends on the field. In computing, a jiffy is typically 1/100th of a second (10 milliseconds) — one tick of the system timer. In physics, Gilbert Newton Lewis defined a jiffy as the time it takes light to travel one centimeter, approximately 33.3 picoseconds (trillionths of a second). In electronics, it refers to one cycle of alternating current (1/60th of a second in the US, 1/50th in the UK). The word entered English as vague slang for "a short time" in the 18th century, then was co-opted by scientists who needed a name for very small intervals.

From Slang to Science

The word "jiffy" first appeared in English in the late 1700s as informal slang meaning "a very short time." Its exact origin is unknown — etymologists have proposed connections to lightning (as in a flash), to thieves' cant, and to various dialectal words, but none of these derivations is confirmed. By the 1800s, "I'll be back in a jiffy" was common colloquial English on both sides of the Atlantic.

The word made the unusual jump from casual slang to scientific terminology in the early 20th century, when physicists and engineers needed short, convenient names for very small time intervals. Rather than inventing entirely new words, they borrowed "jiffy" and gave it precise definitions — different definitions for different contexts, which has created a situation where "a jiffy" can mean anything from 10 milliseconds to 33 trillionths of a second depending on who is talking.

The Computing Jiffy

In computer science, a jiffy is the duration of one tick of the system timer interrupt — the regular heartbeat that drives the operating system's internal clock. On most Linux systems, this is configured at 100 Hz (100 ticks per second), making one jiffy equal to 10 milliseconds. Some systems use 250 Hz or 1000 Hz, making a jiffy 4 milliseconds or 1 millisecond respectively.

The jiffy is fundamental to how operating systems manage time. Process scheduling, timer events, and many other kernel operations are measured in jiffies. When you write code that says "wait for 50 jiffies," you are telling the system to wait for 50 timer ticks — the exact real-time duration depends on the system's configured tick rate.

This is relevant to anyone who has wondered why their Bluetooth speaker keeps disconnecting or why certain computer operations have seemingly arbitrary timing. Under the hood, many of these operations are quantized to jiffy boundaries. A request to sleep for 15 milliseconds on a 100 Hz system will actually sleep for either 10 or 20 milliseconds, depending on where in the current jiffy the request arrives.

The Linux kernel defines the jiffy explicitly in its source code through the CONFIG_HZ parameter. You can check your system's jiffy duration by looking at this configuration value. The choice of tick rate involves a tradeoff: higher rates give more precise timing but consume more CPU time processing interrupts. Lower rates are more efficient but less precise.

The Physics Jiffy

The physics jiffy was defined by the American physical chemist Gilbert Newton Lewis in the 1920s. Lewis (who also coined the word "photon" and developed the concept of electron pair bonds) defined a jiffy as the time required for light to travel one centimeter in a vacuum.

Light travels at approximately 299,792,458 meters per second. One centimeter is 0.01 meters. The time to traverse that distance is:

0.01 / 299,792,458 = approximately 33.3564 picoseconds

A picosecond is one trillionth of a second (10 to the negative 12th). So a physics jiffy is about 33.4 trillionths of a second — a duration so short that light itself can barely move in it.

This definition is elegantly practical. In many physics calculations involving electromagnetic radiation, the centimeter is a natural unit of distance, and having a named time unit corresponding to "one centimeter of light travel" simplifies expressions. Rather than writing "3.34 times 10 to the negative 11 seconds" repeatedly, you can write "one jiffy."

The Electronics Jiffy

In electrical engineering, particularly in the context of alternating current (AC) power, a jiffy refers to the duration of one cycle of the AC waveform. In the United States and other countries using 60 Hz power, this is 1/60th of a second (approximately 16.7 milliseconds). In the UK, Europe, and other regions using 50 Hz power, it is 1/50th of a second (20 milliseconds).

This definition arose because many electronic circuits use the AC power cycle as a timing reference. Before the widespread availability of quartz crystal oscillators, the AC line frequency was one of the most accurate and readily available timing signals. Clocks, timing circuits, and control systems were designed around the AC cycle, making "one jiffy" a natural unit.

The AC line frequency is maintained with considerable precision by power grid operators — in the US, the average frequency over a 24-hour period is kept within a tiny fraction of 60 Hz. This is why older electric clocks (which counted AC cycles to keep time) were surprisingly accurate, and why smart plugs and similar devices sometimes behave oddly if the line frequency fluctuates.

Other Definitions

The jiffy has acquired additional definitions in specific subfields:

In quantum physics, a jiffy is sometimes used to mean the time for light to travel the radius of a hydrogen atom's electron cloud (about 0.53 angstroms), which works out to roughly 1.77 times 10 to the negative 24 seconds — an almost inconceivably short duration.

In animation, particularly in the GIF format, timing between frames is measured in "centiseconds" (hundredths of a second), which are sometimes called jiffies.

In chemistry, Lewis's original definition is sometimes extended or modified for different contexts, though the "light traveling one centimeter" version remains the most standard.

The proliferation of definitions is both a strength and a weakness. The word fills a genuine need — scientists in many fields want a short, memorable name for a very brief time interval. But the lack of a single universal definition means you always need to know which jiffy someone is talking about.

The Broader Lesson

The story of the jiffy illustrates something interesting about how scientific language develops. Scientists are human beings who speak colloquial languages, and they routinely borrow everyday words and give them precise technical meanings. "Work" in physics means something specific (force times distance) that differs from its everyday meaning. "Stress" in engineering (force per unit area) differs from its psychological meaning. "Culture" in microbiology (a colony of microorganisms) differs from its anthropological meaning.

"Jiffy" followed the same path, from vague slang to precise measurement. The difference is that it acquired multiple precise meanings in different fields simultaneously, making it perhaps the most ambiguous unit of measurement in science — a word that is both perfectly defined and hopelessly imprecise, depending on context.

So the next time someone says they will be "back in a jiffy," you are within your rights to ask: a computing jiffy, a physics jiffy, or an electronics jiffy? Though if you value the friendship, you might want to just smile and wait the 10 milliseconds. Or the 33 picoseconds. Or the 16.7 milliseconds.