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

Upon arrival, the Fire Department located five separate spot fires, all about five feet in diameter.

From Los Angeles Times ● Sep. 12, 2026

But because the diameter of the Moon is 400 times less than the Sun's, they appear exactly the same size in our sky.

From BBC ● Aug. 12, 2026

And it has to come from a place that looks localized to the mouth, and it has to be the same diameter of the actor’s mouth.

From Los Angeles Times ● Aug. 6, 2026

When the Kimberly-Clark team studied it, the length of its fibers and their slender diameter stood out.

From The Wall Street Journal ● Aug. 3, 2026

He started from scratch, replacing Edlefsen’s lumps of metal and sealing wax with a vacuum chamber fashioned from brass in the shape of a flat cylinder four inches in diameter.

From "Big Science" by Michael Hiltzik

An artist's impression of a red dwarf with a white dwarf binary companion behind, with the diameters shown to scale.

From BBC ● Jul. 14, 2026

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

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

From New York Times ● Apr. 27, 2024

"From several micron diameters down to hundreds of nanometers, in a variety of materials, we're able to examine particle behavior, such as how they scatter or how their structures change upon impact."

From Science Daily ● Dec. 5, 2023

The pitch of teeth may be selected so that with the given number of teeth the diameters of the wheels will be that required.

From Modern Machine-Shop Practice, Volumes I and II by Joshua Rose

Related Words

• width