ra·​di·​us | \ ˈrā-dē-əs How to pronounce radius (audio) \
plural radii\ ˈrā-​dē-​ˌī How to pronounce radii (audio) \ also radiuses

Definition of radius

1 : a line segment extending from the center of a circle or sphere to the circumference or bounding surface
2a : the bone on the thumb side of the human forearm also : a corresponding part of vertebrates above fishes
b : the third and usually largest vein of an insect's wing
3a : the length of a radius a truck with a short turning radius
b : the circular area defined by a stated radius
c : a bounded or circumscribed area
4 : a radial part
5 : the distance from a center line or point to an axis of rotation

Examples of radius in a Sentence

Measure the radius of the circle. a radius of 10 inches The new museum is expected to draw people from a wide radius. There are three restaurants within a one-block radius of our apartment.
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Recent Examples on the Web

For those unlucky enough to live outside the radius of a major city, Bond has a new project with JW Anderson, advertising the label’s new Keyts bag. Steff Yotka, Vogue, "Justin Vivian Bond Would Like to Sell You a Bag," 20 Mar. 2019 Multiply width by length for square or rectangular beds—or for round beds, multiply the radius (distance from the middle to the edge of the bed) by itself, and then multiple that total by 3.14. Arricca Sansone, Country Living, "How Much Mulch Do I Need? Here's the Easiest Way to Find Out," 6 Feb. 2019 This season, the circle at the first 24-hour forecast point will have a radius of 43 nautical miles (49 land miles), compared with 67 nautical miles (77 land miles) in 2008. Renée Rigdon And Arian Campo-flores, WSJ, "How to Tell if a Hurricane Is Headed Your Way," 11 Sep. 2018 There are limits to how well an MRI or, in earlier studies, x-rays could image bubbles that, on average, have a radius of just 200 micrometers—or 200 one-thousandths of a millimeter. Jeffrey Kluger, Time, "Why Does Cracking Your Knuckles Make So Much Noise? Science Finally Has an Answer," 29 Mar. 2018 These Rydberg atoms can have a radius as large as a micrometer, about a thousand times larger than their normal size. Chris Lee, Ars Technica, "How a string of giant atoms might bring quantum computers to physics labs," 8 Mar. 2018 That means that the devices send out electromagnetic waves that can be picked up in a radius of around 32 feet (10 meters) of the wearer before returning to the device. David Grossman, Popular Mechanics, "Scientists Design a Network That Lives Inside Your Body," 13 Mar. 2019 But, like any DIY project, there are a few tips worth knowing that can save you a lot of headaches — and trips to Target, HomeGoods, and well, every hardware store in a 20-mile radius. Candace Braun Davison, House Beautiful, "HGTV's Jasmine Roth Has An Awesome Alternative To The Basic Gallery Wall," 28 Aug. 2018 While McDonald's does not have any other restaurants in Brown Deer, the company does have three other restaurants in a 5-mile radius, including those at 7170 N. Teutonia Ave. Jeff Rumage, Milwaukee Journal Sentinel, "McDonald's in Brown Deer is being demolished, and there's no plan to replace it," 4 June 2018

These example sentences are selected automatically from various online news sources to reflect current usage of the word 'radius.' 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 radius

1578, in the meaning defined at sense 1

History and Etymology for radius

Latin, ray, radius

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Statistics for radius

Last Updated

4 May 2019

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Time Traveler for radius

The first known use of radius was in 1578

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English Language Learners Definition of radius

technical : a straight line from the center of a circle or sphere to any point on the outer edge also : the length of this line
: an area that goes outward in all directions from a particular place
medical : the bone in the lower part of your arm on the same side as your thumb


ra·​di·​us | \ ˈrā-dē-əs How to pronounce radius (audio) \
plural radii\ -​dē-​ˌī \

Kids Definition of radius

1 : a straight line extending from the center of a circle to the outside edge or from the center of a sphere to the surface
2 : an area that extends in all directions from a place Most students live within a radius of five miles from the school.
3 : the bone on the thumb side of the arm between the wrist and the elbow


ra·​di·​us | \ ˈrād-ē-əs How to pronounce radius (audio) \
plural radii\ -​ē-​ˌī How to pronounce radii (audio) \ also radiuses

Medical Definition of radius

: the bone on the thumb side of the human forearm or on the corresponding part of the forelimb of vertebrates above fishes that in humans is movably articulated with the ulna at both ends so as to permit partial rotation about that bone, that bears on its inner aspect somewhat distal to the head a prominence for the insertion of the biceps tendon, and that has the lower end broadened for articulation with the proximal bones of the carpus so that rotation of the radius involves also that of the hand

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More from Merriam-Webster on radius

Spanish Central: Translation of radius

Nglish: Translation of radius for Spanish Speakers

Britannica English: Translation of radius for Arabic Speakers

Britannica.com: Encyclopedia article about radius

Comments on radius

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


something valued as if it were money

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