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

And every stripper that lived, not even just Los Angeles, within a hundred-mile radius was there. Matt Wake | Mwake@al.com, al.com, "An ’80s glam-metal survivor tells all," 9 July 2019 Playa Del Rey, in the sprawling Los Angeles basin—contains more than half a million residents within a five-mile radius. Larry Buhl, The New Republic, "The Toxic-Gas Catastrophe Hiding Beneath Your Home," 8 July 2019 Rattlesnakes often stay within a one-mile radius of their birthplaces for their entire lives, which can span 15 to 20 years, according to the Department of Fish and Wildlife. Tom Stienstra, SFChronicle.com, "Rattlesnake season is here. Be prepared.," 28 June 2019 The department's investigators are asking residents and businesses within a half-mile radius of the bar to check their backyards, alleyways and properties for anything suspicious or strange, including clothing and weapons. Jack Mccordick, Milwaukee Journal Sentinel, "Racine police announce increased reward for suspect who shot an off-duty officer," 24 June 2019 In the letter, Palter requested that 70 percent of market-rate units be built off campus, within a six-mile radius, and 70 percent of affordable units be built on campus. Thy Vo, The Mercury News, "Stanford tells Santa Clara County to get real about its housing demands," 11 June 2019 The neighborhood is growing, with 638 housing lots on the market within a three-mile radius of the Steelyard, Hersker said. Riley Murdock, azcentral, "New south Chandler development will feature Pita Jungle, other businesses," 7 June 2019 Hennessy said Great Wolf Lodges typically draw guests from a three- to four-hour drive radius. Melissa Yeager, azcentral, "Great Wolf Lodge Arizona moves up its opening date, adds twisty Diamondback Drop slide," 11 June 2019 Researchers studied wind patterns to find a source of the microplastics collected, but found none within a 60-mile radius of the region—which is sparsely populated and without industrial, commercial, or large agricultural activities. Stephen Leahy, National Geographic, "Microplastics are raining down from the sky," 15 Apr. 2019

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

13 Jul 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

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