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

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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

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Other Word Forms

Derived Forms

• millimetric
  adjective

Etymology

Origin of millimeter

First recorded in 1800–10; from French millimètre; see 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

While a single 300 millimeter silicon wafer normally produces a few dozen chips, Cerebras uses the entire wafer for one chip, about the size of a dinner plate.

From Barron's • May 15, 2026

In fact, a projector or display with a resolution of 1920 x 1080 pixels could fit within an area of just one square millimeter.

From Science Daily • Mar. 4, 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

The decision should be carefully considered, she added: “This is a procedure where a millimeter makes a tremendous difference—it could really change how you look.”

From The Wall Street Journal • Jan. 1, 2026

“It’s an integrated circuit on a sphere of silicon about one millimeter in diameter,” Alex said.

From "Stormbreaker" by Anthony Horowitz