A millisecond is one thousandth of a second. If you divided a single second into 1,000 tiny equal pieces, each piece would be one millisecond. Scientists and engineers write it as “ms” for short.

A millisecond passes so quickly that you can't even blink in that time. A blink takes about 100 to 150 milliseconds. When you click a mouse, the computer registers it in just a few milliseconds. Professional sprinters might win or lose a race by milliseconds, differences so small that only electronic timing can measure them accurately.

Milliseconds matter most in situations requiring extreme precision: computer processors perform billions of operations per second, with each step measured in milliseconds or even smaller units. Video game players notice if their reaction time is off by 50 milliseconds. Seismologists studying earthquakes track tremors that last only milliseconds.

In everyday conversation, when someone says “just a millisecond,” they usually mean “an incredibly short time,” even if they don't mean exactly one thousandth of a second.