diameter

American  

[dahy-am-i-ter] / daɪˈæm ɪ tər /

noun

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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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

“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

They are considered harmless to humans and are predominantly found sitting in large webs often more than a metre in diameter.

From BBC • Feb. 10, 2026

Similarly, supermarket apples are typically around three inches in diameter, wild apples only one inch.

From "Guns, Germs, and Steel: The Fates of Human Societies" by Jared M. Diamond

Related Words

• width