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

It is feasible that the winning team will have had to travel a distance the equivalent to the diameter of the Earth.

From BBC ● Jun. 11, 2026

A traditional arthroscope measures about 4 millimeters in diameter.

From The Wall Street Journal ● May 26, 2026

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

From Los Angeles Times ● Apr. 10, 2026

Saturn's magnetosphere is enormous, extending to more than 10 times the planet's diameter.

From Science Daily ● Apr. 3, 2026

The smallest ones were sixteen inches in diameter and twenty feet long.

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

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

It is used in distillations, to give more space to elastic vapors, to increase the length of the neck of a retort, or to unite two vessels whose openings have different diameters.

From Webster's Unabridged Dictionary by Webster, Noah

Related Words

• width