diameter

American  

[dahy-am-i-ter] / daɪˈæm ɪ tər /

noun

diameters plural

1. Geometry.

   1. a straight line passing through the center of a circle or sphere and meeting the circumference or surface at each end.

   2. a straight line passing from side to side of any figure or body, through its center.

2. the length of such a line.

3. the width of a circular or cylindrical object.

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diameter British  

/ daɪˈæmɪtə /

noun

1. 1. a straight line connecting the centre of a geometric figure, esp a circle or sphere, with two points on the perimeter or surface

   2. the length of such a line

2. the thickness of something, esp with circular cross section

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diameter Scientific  

/ dī-ăm′ĭ-tər /

1. A straight line segment that passes through the center of a circle or sphere from one side to the other.

2. The length of such a line segment.

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diameter Cultural  

1. A straight line passing through the center of a figure, especially a circle or sphere, and joining two opposite points on its circumference.

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Other Word Forms

Noun Inflected Forms

• diameters
  plural

Etymology

Origin of diameter

First recorded in 1350–1400; Middle English diametre, from Old French, from Latin diametros, from Greek diámetros “diagonal, diameter,” from dia- dia- + -metros, derivative of métron meter ¹

Explanation

It's good to know where the diameter of a circular object is if you have to split a cookie between two 5-year-old kids. Think of a round chocolate chip cookie. The diameter of the cookie is a line that divides the cookie into two equal halves, through the center of the cookie. The diameter of a circle or sphere is a line that passes through the center and starts and ends and two opposite points on its perimeter or edge. The diameter is also the length of that line. In math class, teachers often ask you to use the diameter of a circle to find its area. This is a good skill to have if you are trying to figure out whether the 14 inch or 16 inch pizza is a better deal.

Example Sentences

See Examples For:

diameter diameters

The heat created by its beams can seal blood vessels just 2mm in diameter, about 5-6cm below the skin.

From BBC ● Jul. 10, 2026

These fibrils were then combined with potassium hydroxide and formed into porous beads measuring about half a centimeter to one centimeter in diameter.

From Science Daily ● Jun. 11, 2026

In January, a 72-inch diameter pipe ruptured, spewing 243 million gallons of raw sewage into the Potomac River in one of the worst wastewater spills in U.S. history.

From The Wall Street Journal ● Apr. 20, 2026

“I think it was a 10-foot diameter ... everything kind of flowed back from that.”

From Los Angeles Times ● Apr. 10, 2026

But “from the late 1990s until now—it’s still going on—the ground surface has lifted up about a foot over about a 12-mile diameter circle,” said Major.

From "Mountain of Fire" by Rebecca E. F. Barone

Which is to say, Porsche invented an entirely new flat-six—different bore diameters and everything—just to clear 11 centimeters under the engine cover.

From The Wall Street Journal ● Oct. 30, 2025

Under extended exposure, the process produced nearly perfect nanodiamonds with a cubic crystal structure and diameters up to 10 nanometers, along with the release of hydrogen gas.

From Science Daily ● Oct. 29, 2025

Hailstones, some with diameters of around 2 inches, also fell over parts of the city.

From New York Times ● Apr. 27, 2024

PM 2.5 refers to the level of tiny, hazardous particles - with diameters that are 2.5 micrometres or smaller - that can enter bloodstreams though the lungs.

From BBC ● Mar. 6, 2024

The halo when complete consists of two large circles whose diameters are constant, 45 and 92 degrees.

From Reading the Weather by Longstreth, Thomas Morris

Related Words

• width