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

• 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

The chip is giant, taking up an entire 300 millimeter silicon wafer, almost a foot long across the chip’s diagonal line.

From Barron's • May 11, 2026

On average, the gloves introduced around 2,000 false positive signals per square millimeter.

From Science Daily • Mar. 29, 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

When I was exactly halfway across the Fifty-ninth Street Bridge, I thought about how a millimeter behind me was Manhattan and a millimeter in front of me was Queens.

From "Extremely Loud and Incredibly Close" by Jonathan Safran Foer