In Physics, a 'Jiffy' Is an Actual Measurement: One Hundredth of a Second

From Slang to Specification

The English language is full of words that mean "a short time" — moment, instant, flash, tick. "Jiffy" has been among them since at least the late 18th century, always informal and always imprecise. It carries a sense of confident brevity: "I'll be with you in a jiffy" implies that the wait will be short but makes no commitment to exactly how short.

Scientists and engineers have a different relationship with language than the rest of us. Where everyday speech tolerates productive vagueness, technical communication demands precision. The impulse to assign an exact value to "jiffy" — to rescue it from imprecision — has led to at least two distinct technical definitions in different fields. In certain electrical engineering and computing contexts, a jiffy represents 1/100th of a second, equal to 10 milliseconds.

Why 10 Milliseconds?

The definition of a jiffy as 1/100th of a second has a practical origin in the alternating current (AC) power systems that power most electrical grids. In countries using 50 Hz AC power (most of Europe, Africa, and Asia), the current completes 50 full cycles per second, meaning each cycle lasts 20 milliseconds — or 2 jiffies. In countries using 60 Hz power (North America, parts of South America), each cycle lasts approximately 16.67 milliseconds.

The 1/100th of a second definition (10 milliseconds) provides a convenient common reference unit in contexts where timing needs to align with the frequency of AC power systems. Many early electronic devices were designed around the rhythm of the power supply, making a unit tied to that frequency naturally convenient.

In software and operating system design, a jiffy is often defined differently — as one period of the system clock, which may be 1, 4, or 10 milliseconds depending on the system configuration. But the 10-millisecond definition has persisted in enough technical literature that NIST and other standards bodies acknowledge it as one formal usage.

The Curious Life of an Informal Word

The story of "jiffy" illustrates something characteristic about how scientific vocabulary develops. Rather than always inventing new words for new concepts, scientists frequently borrow existing words and assign them precision. The borrowed word carries useful connotations — "jiffy" implies smallness and quickness — while the formal definition adds the specificity that technical communication requires.

The physicist Gilbert N. Lewis famously proposed yet another definition in 1927: the time for light to travel one centimeter, approximately 33 picoseconds. This ultra-short jiffy never gained wide acceptance, but it represents the same intellectual move — taking a casual word and asking "what if we made this exact?"

The result is that "jiffy" now lives in at least three parallel worlds simultaneously: the casual world of everyday speech, where it means simply "soon"; the computing world, where it means one system clock period; and certain engineering contexts, where it means 10 milliseconds. The word remains useful in all three, which is itself a small testament to the flexibility of language when precision is layered onto familiarity.