diameter
Americannoun
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Geometry.
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a straight line passing through the center of a circle or sphere and meeting the circumference or surface at each end.
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a straight line passing from side to side of any figure or body, through its center.
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the length of such a line.
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the width of a circular or cylindrical object.
noun
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a straight line connecting the centre of a geometric figure, esp a circle or sphere, with two points on the perimeter or surface
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the length of such a line
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the thickness of something, esp with circular cross section
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A straight line segment that passes through the center of a circle or sphere from one side to the other.
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The length of such a line segment.
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.
Vocabulary lists containing diameter
Pi Day Vocabulary
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The ACT Math Test: Geometry, List 1
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List 2
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Example Sentences
Examples are provided to illustrate real-world usage of words in context. Any opinions expressed do not reflect the views of Dictionary.com.
The bass society limits sonar users to one transducer, while viewing screens can’t have a diameter of more than 55 inches.
From The Wall Street Journal • May 5, 2026
"It weighs 8.2kg, measures nearly 30cm in diameter and stands about 46cm tall - almost four times the size of a typical astrolabe from 17th Century India," said Carter.
From BBC • Apr. 25, 2026
“I think it was a 10-foot diameter ... everything kind of flowed back from that.”
From Los Angeles Times • Apr. 10, 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
One highly effective campaign recently asked you whether you could poke your finger through the hole, approximately the diameter of a golf ball, cut in the front cover of the leaflet.
From "Words Like Loaded Pistols" by Sam Leith
Definitions and idiom definitions from Dictionary.com Unabridged, based on the Random House Unabridged Dictionary, © Random House, Inc. 2023
Idioms from The American Heritage® Idioms Dictionary copyright © 2002, 2001, 1995 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company.
