di·​am·​e·​ter | \ dī-ˈa-mə-tər How to pronounce diameter (audio) \

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

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Other Words from diameter

diametral \ dī-​ˈa-​mə-​trəl How to pronounce diameter (audio) \ 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.
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Recent Examples on the Web Inside a large safety cage, robot arms wheel around grasping circular pieces of metal, each about the diameter of a dinner plate, from a conveyor and slot them together. Will Knight, Wired, "Ford's Ever-Smarter Robots Are Speeding Up the Assembly Line," 28 Apr. 2021 The maximum size of a raindrop on Earth is 11.18 millimeters -- just over the diameter of an Advil tablet -- while the maximum size on Titan is more than twice that at 29.96 millimeters, which is larger than a penny. Ashley Strickland, CNN, "What alien raindrops on other planets have in common with rain on Earth," 13 Apr. 2021 The second number in a binoculars’ description is the diameter of the objective lenses in millimeters. Daisy Kelly, chicagotribune.com, "The best Celestron binoculars," 11 Apr. 2021 Per a statement, the artworks slated to go on view range in size from the diameter of a one-pound coin (roughly the same as an American quarter) to just under eight inches. Isis Davis-marks, Smithsonian Magazine, "Dollhouse-Sized Exhibition Will Showcase Mini Creations by Art World Giants," 1 Apr. 2021 Likewise, with the dado blade raised to the depth of the groove, mark the diameter of the saw blade on the saw table (the position of the front tooth and the back tooth). Joseph Truini, Popular Mechanics, "Double Duty: A Trellis Planter for Plants and Privacy," 27 Mar. 2021 For aerial spraying, planes are equipped with a high-pressure applicator that atomizes the product into billions of tiny droplets, 50 microns across, or less than the diameter of a human hair. Brian Maffly, The Salt Lake Tribune, "Air Force planning an aerial assault on mosquitoes near the Great Salt Lake," 8 Mar. 2021 The planting hole should be a few inches wider than the diameter of the rootball and just deep enough so that the flare at the base of the trunk will be one or two inches above the surrounding soil level. Adrian Higgins, Washington Post, "The gardener’s guide to redbuds," 4 Mar. 2021 The 18-inch diameter of the ring gives you more light to work with, and the hot shoe adapter works with most DSLR models to hold your camera in place in the ring. Popular Science, "Ring lights that make you look great on camera," 12 Jan. 2021

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.

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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

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Time Traveler for diameter

Time Traveler

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

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Statistics for diameter

Last Updated

3 May 2021

Cite this Entry

“Diameter.” Merriam-Webster.com Dictionary, Merriam-Webster, https://www.merriam-webster.com/dictionary/diameter. Accessed 14 May. 2021.

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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 How to pronounce diameter (audio) \

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 How to pronounce diameter (audio) \

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

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