ra·​di·​us | \ˈrā-dē-əs \
plural radii\ˈrā-​dē-​ˌī \ 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

Most of the migrants stay within a three-block radius of the sports complex that faces the towering metal walls topped with barbed wire at the U.S.-Mexico border. Julie Watson, The Seattle Times, "Caravan migrants weigh options after court blocks Trump ban," 20 Nov. 2018 Almost everything comes from within a three-block radius. Paul Jablow, Philly.com, "A South Street apartment's journey from eclectic to psychedelic and back again," 19 June 2018 Sayban said most visitors and members live within a 10-mile radius, but some come from as far as Gilbert and Prescott. Georgann Yara, azcentral, "Soccer, sense of community boost Arizona Sports Complex in Glendale," 29 May 2018 There are some 800 languages spoken within the 10-mile radius of New York City, which is more than 10 percent of the world’s estimated 7,099 languages. National Geographic, "The Race to Save the World's Disappearing Languages," 16 Apr. 2018 Though some can scan up to a mile radius, most are fairly localized. Lily Hay Newman, WIRED, "DC's Stingray Mess Won't Get Cleaned Up," 6 Apr. 2018 Free delivery with the purchase of 15 bags within a 10-mile radius. Kerry Plummer, Howard County Times, "Stay awake for Marriotts Ridge's production [Marriottsville/Sykesville/Woodstock]," 15 Mar. 2018 States with nuclear reactors and populations within a 10-mile radius of the reactors stockpile potassium iodide to distribute in case of an emergency, according to the Nuclear Regulatory Commission. NBC News, "Anti-radiation drug sales skyrocket after ‘nuclear button’ tweet," 11 Jan. 2018 Authorities evacuated a half-mile radius around the center of the explosion. Todd Richmond, chicagotribune.com, "Firefighter killed, a dozen injured in gas explosion and fire near Madison, Wisconsin," 11 July 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

30 Nov 2018

Look-up Popularity

Time Traveler for radius

The first known use of radius was in 1578

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English Language Learners Definition of radius

: 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 \
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 \
plural radii\-​ē-​ˌī \ 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).


the figure or shape of a crescent moon

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