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

Aside from studying the plume, the agency is requiring the Air Force to sample all water wells within a 4-mile (6-kilometer) radius of the southeastern corner of the base. Susan Montoya Bryan, The Seattle Times, "New Mexico concerned about contamination at US air base," 16 Oct. 2018 That equipment can provide a wide radius of service, but is costly. Sarah Krouse, WSJ, "Fiber Damage Vexes Verizon After Hurricane Michael," 14 Oct. 2018 The aircraft has a combat radius of 300 nautical miles, can reach speeds up to 367 mph and hits ranges up to 720 nautical miles. Kris Osborn, Fox News, "New Air Force light attack plane moves quickly toward combat," 21 Aug. 2018 The 39-foot turning circle is large, but some family sedans have a foot-wider radius. Mark Maynard, sandiegouniontribune.com, "2018 Buick Enclave Avenir: French flair for working-class luxury," 11 May 2018 Officials said homes over a radius of nearly 1.2 miles were submerged. Tom Odula, BostonGlobe.com, "At least 44 killed as dam bursts in Kenya, officials say," 10 May 2018 Officials said homes over a radius of nearly 1.2 miles were submerged. USA TODAY, "At least 44 killed as dam bursts in Kenya and sweeps away hundreds of homes," 10 May 2018 Social media is abuzz that the casting directors are seeking prospects within a one-hour radius of Kansas City. Matt Campbell, kansascity, "'Queer Eye' looks to Kansas City to cast its next season | The Kansas City Star," 25 Apr. 2018 The 30-million-old year star is nine times the mass of the sun and has a radius that is 200-times larger, according to the researcher. Lauren Castle, azcentral, "Lowell Observatory researcher discovers 'runaway' star 200,000 light years from Earth," 3 Apr. 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

14 Dec 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

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