di·​am·​e·​ter | \dī-ˈa-mə-tər \

Definition of diameter 

1 mathematics : a chord (see chord entry 3 sense 2) passing through the center of a figure or body

2 mathematics : the length of a straight line through the center of an object or space the diameter of a circle dug a hole nearly four feet in diameter

3 physics : a unit of enlargement used with a number to indicate magnification by a lens or optical system an object one millimeter wide magnified 40 diameters appears 40 millimeters wide

Keep scrolling for more

Other Words from diameter

diametral \-​ˈa-​mə-​trəl \ adjective

Examples of diameter in a Sentence

The dotted line indicates the diameter of the circle. What is the diameter of the tree trunk? Dig a hole that's two feet deep and three feet in diameter.
See More

Recent Examples on the Web

Specifically, Loeb pegged it as being as small as 20 meters in diameter. Rob Reid, Ars Technica, "Nailing down the nature of ‘Oumuamua—it’s probably a comet, but…," 29 Nov. 2018 And fine particulates, classified as 2.5 microns or less in diameter, are just one type of pollution. Umair Irfan, Vox, "How improving air quality could add years to people’s lives around the world.," 21 Nov. 2018 At the top of the dome, a window 30 feet in diameter looks out onto sky. Liz Stinson, Curbed, "James Turrell’s new Skyspace is nestled into the Austrian mountains," 1 Oct. 2018 Johnson cites the daunting NASA estimates: There are roughly 25,000 Didymoon-scale asteroids, 230,000 asteroids in the 150-foot range, and millions more of diameters of 100 feet or less. NBC News, "Space scientists have big plans to avert an asteroid apocalypse," 6 Apr. 2018 Based upon what Musk said in 2017, the spacecraft for which this tooling was made has a diameter of 9 meters and encompasses six engines, propellant tanks, and a large payload area that could carry many satellites into space or carry people to Mars. Eric Berger, Ars Technica, "Elon Musk shows off a new tooling for the BFR spaceship," 9 Apr. 2018 The sub-classifications of A to C are based on the width or diameter of your wave, curl, or coil pattern. Joane Amay, Allure, "How to Figure Out Your Curl Type," 19 Jan. 2018 In Melbourne, Australia, 4% of the sun’s diameter will be covered, which means only a sliver of sun will be darkened by the eclipse. Gabby Raymond, Time, "A Partial Solar Eclipse Is Coming on Friday the 13th. That Hasn't Happened in 40 Years," 10 July 2018 Campfires with a diameter of less than 3 feet are allowed but not advised, according to the Alaska Division of Forestry, which suspended burning in the Mat-Su. Devin Kelly, Anchorage Daily News, "Dry weather keeps burn bans in effect for Anchorage and Mat-Su," 6 July 2018

These example sentences are selected automatically from various online news sources to reflect current usage of the word 'diameter.' Views expressed in the examples do not represent the opinion of Merriam-Webster or its editors. Send us feedback.

See More

First Known Use of diameter

14th century, in the meaning defined at sense 1

History and Etymology for diameter

Middle English diametre, from Middle French, from Latin diametros, from Greek, from dia- + metron measure — more at measure

Keep scrolling for more

Learn More about diameter

Statistics for diameter

Last Updated

9 Dec 2018

Look-up Popularity

Time Traveler for diameter

The first known use of diameter was in the 14th century

See more words from the same century

Keep scrolling for more

More Definitions for diameter



English Language Learners Definition of diameter

: a straight line from one side of something (such as a circle) to the other side that passes through the center point

: the distance through the center of something from one side to the other


di·​am·​e·​ter | \dī-ˈa-mə-tər \

Kids Definition of diameter

1 : a straight line that runs from one side of a figure and passes through the center Measure the diameter of the circle.

2 : the distance through the center of an object from one side to the other : thickness the diameter of a tree trunk


di·​am·​e·​ter | \dī-ˈam-ət-ər \

Medical Definition of diameter 

1 : a unit of magnification for an optical instrument equal to the number of times the linear dimensions of an object are apparently increased a microscope magnifying 60 diameters

2 : one of the maximal breadths of a part of the body the transverse diameter of the inlet of the pelvis

Keep scrolling for more

More from Merriam-Webster on diameter

Rhyming Dictionary: Words that rhyme with diameter

Spanish Central: Translation of diameter

Nglish: Translation of diameter for Spanish Speakers

Britannica English: Translation of diameter for Arabic Speakers

Comments on diameter

What made you want to look up diameter? Please tell us where you read or heard it (including the quote, if possible).


a typical or ideal example

Get Word of the Day daily email!

Test Your Vocabulary

Find the Cousins

  • a-large-tree-with-many-branches
  • Which pair shares a common word ancestor?
Spell It

Can you spell these 10 commonly misspelled words?


Test Your Knowledge - and learn some interesting things along the way.


Love words? Need even more definitions?

Subscribe to America's largest dictionary and get thousands more definitions and advanced search—ad free!