millimeter

American  

[mil-uh-mee-ter] / ˈmɪl əˌmi tər /

especially British, millimetre

noun

1. a unit of length equal to one thousandth of a meter and equivalent to 0.03937 inch. mm

---

millimeter Scientific  

/ mĭl′ə-mē′tər /

1. A unit of length in the metric system equal to 0.001 meter.

2. See Table at measurement

---

Other Word Forms

• millimetric adjective

Etymology

Origin of millimeter

First recorded in 1800–10; from French millimètre; milli-, meter ¹

Explanation

A millimeter is a tiny unit of measurement. The thickness of a dime is a little more than a millimeter, and there are a thousand millimeters in a meter. If you're using metric units to measure the length of something, you'll be dealing in meters and variations on meters, including millimeters. Look at a ruler that includes centimeters, the units of metric measurement that are closest in length to inches. The smallest units marked on the ruler are millimeters — there are ten of them in each centimeter. Although the word millimeter sounds like "million," it actually means "thousandth of a meter."

Example Sentences

The entire array fits on a chip smaller than a millimeter, making it suitable for compact wireless access points and potentially small enough to integrate into devices such as smartphones.

From Science Daily • Apr. 2, 2026

In this environment, the sediment layer grows at a rate of just one thousandth of a millimeter per year.

From Science Daily • Feb. 2, 2026

In the first five days, forensic experts gingerly removed earth, millimeter by millimeter, recovering 3,463 bone fragments.

From Barron's • Nov. 29, 2025

His record-breaking set up was quite typical: a Rose commuter bike with a flat bar and slick, 50 millimeter width tires, standard pressure.

From The Wall Street Journal • Nov. 13, 2025

Start with a millimeter, which is a line this long: –.

From "A Short History of Nearly Everything" by Bill Bryson