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

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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

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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 break occurred on a water main pipe that’s about 8 inches in diameter, Workman said, and affected about 30 customers in the area.

From Los Angeles Times ● Oct. 14, 2026

For the Magic 8 Ball, it was figuring out how the answer block could float without developing air bubbles, since his version is about an inch in diameter.

From The Wall Street Journal ● Oct. 1, 2026

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

From Los Angeles Times ● Sep. 12, 2026

Roman has a 7.9-foot primary mirror, the same diameter as the Hubble Space Telescope's, along with two major scientific instruments.

From Science Daily ● Aug. 28, 2026

Protons and neutrons are about 10~12 centimeters in diameter.

From "Innumeracy: Mathematical Illiteracy and Its Consequences" by John Allen Paulos

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

Astronomers these days are really interested in sub-Neptune planets, meaning those with diameters larger than Earth but smaller than Neptune.

From Salon ● May 31, 2025

They have up to 24 arms and can grow to diameters of more than three feet.

From New York Times ● Mar. 6, 2024

Furthermore, different diameters of roller require different forms of cam to accomplish the same motion, or, in other words, with a given cam the action will vary with different diameters of roller.

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

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