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

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

From Los Angeles Times ● Apr. 10, 2026

The dragging of ship anchors, a main way cables get taken out, can damage even armored cables, which are wrapped in steel wire but still roughly the diameter of a ping-pong ball.

From The Wall Street Journal ● Apr. 9, 2026

With a steady hand, she manoeuvred a huge wind turbine section, five metres in diameter, across the yard.

From Barron's ● Mar. 7, 2026

This layer leaves only a circular opening with a diameter of 200 nanometers at the center.

From Science Daily ● Mar. 4, 2026

The club itself was a massive cobalt blue sphere, three kilometers in diameter, floating thirty meters off the ground.

From "Ready Player One: A Novel" by Ernest Cline

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

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 following thicknesses for the respective diameters may be considered safe for castings that have not been reheated after casting.

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

Related Words

• width