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 Assuming a realistic internal strength provided largely by gravity and friction, survival is only an option for very small bodies with a radius below a half-kilometer, or at extreme distances, where the body is well beyond Neptune. John Timmer, Ars Technica, "When the Sun expands, it will trash all the asteroids," 18 Feb. 2020 Warren and its 3,600 service members help protect a nuclear arsenal of 150 ICBM missiles spread over a three-state radius that borders Colorado, where marijuana is legal for recreational use. Abraham Mahshie, Washington Examiner, "Airmen at Wyoming missile base relieved of security duties following alleged marijuana use," 11 Feb. 2020 There are many others — an 1843 letter written by Santa Anna; architectural plans for the San Antonio de Valero Mission, now known as the Alamo; and the original map of the city that details a 1-mile radius from San Fernando Cathedral. ExpressNews.com, "Ayala: San Antonio’s official record-keeper has become a champion of its most historic documents," 8 Feb. 2020 Taurus claims the bigger sight radius afforded by the longer barrel tightens groups considerably. Joseph Albanese, Field & Stream, "Best Cheap Guns of SHOT Show 2020," 7 Feb. 2020 Bringing a complete package, Mayer threatens defenses downfield and for contested grabs with his athleticism, wide catch radius and body control. Carter Karels, Indianapolis Star, "Ranking the 17 members of Notre Dame football's 2020 recruiting class," 6 Feb. 2020 In the aftermath, more than 100,000 people fled a 12-mile radius around the plant. Laura Mallonee, Wired, "The Eerie Repopulation of the Fukushima Exclusion Zone," 3 Feb. 2020 Midband or Sub 6 GHz networks will try to offer the best of both worlds: faster speeds and larger coverage areas, five to six times the speed of LTE with a couple mile radius service area. Arizona Republic, azcentral, "Not all wireless carriers' 5G services perform the same – here's why," 13 Jan. 2020 Before the court blocked it, the Texas law also required physicians performing abortions to have admitting privileges at hospitals within 30 mile radiuses. Abby Vesoulis, Time, "Over 200 Members of Congress Urge Supreme Court to 'Reconsider' Abortion Rights Under Roe v. Wade. Pro-Choice Advocates Say They're Ignoring Public Opinion," 3 Jan. 2020

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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Time Traveler for radius

Time Traveler

The first known use of radius was in 1578

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Statistics for radius

Last Updated

23 Feb 2020

Cite this Entry

“Radius.” Merriam-Webster.com Dictionary, Merriam-Webster, https://www.merriam-webster.com/dictionary/radius. Accessed 26 Feb. 2020.

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More Definitions for radius


How to pronounce radius (audio)

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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