ra·​di·​us | \ ˈrā-dē-əs How to pronounce radius (audio) \
plural radii\ ˈrā-​dē-​ˌī How to pronounce radius (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 Borders closed, and movement within the city was restricted to a one-kilometer radius from home (enforced through the permission-slip tactic). Washington Post, "In a gray, empty Paris, this corner shop’s colorful posters transport you wherever you want to go," 19 Feb. 2021 It’s designed for accuracy down to a two-mile radius. oregonlive, "Lots of snow is heading into the Portland area; our new map shows how much you can expect in the next 48 hours," 11 Feb. 2021 While inventory is scarce across the nation, not being tied to a 20-mile radius from the office opens up your options considerably. Elizabeth Renter Nerdwallet, Star Tribune, "What hopeful home buyers will need to succeed in 2021," 30 Jan. 2021 But increasingly, that treatment area has more than doubled, to a 500-mile radius. NBC News, "'Help is on the way': How the U.S. will turn the tide of the pandemic in 2021," 11 Dec. 2020 Liberty only went so far, though — they were only allowed to venture in a 20-mile radius of campus, and couldn’t buy or drink alcohol or eat indoors. Phillip Jackson, baltimoresun.com, "Maryland’s college bars and restaurants offer students an escape, but not from the masks and social distancing of the pandemic," 17 Sep. 2020 In Miami-Dade, mobile units will drive into the area’s minority communities to inoculate volunteers, and one center will use digital advertising on social media to target a 20-mile radius. Cindy Krischer Goodman, sun-sentinel.com, "South Florida a hotspot for COVID-19 -- and for vaccine trials," 6 Aug. 2020 The map can predict snowfall totals for the next 48 hours for any address, to a radius of about 2 miles. Jane Morice | Jmorice@cleveland.com, cleveland, "OH weather: Lake effect snow begins to fall Sunday; find out how much you may get," 8 Feb. 2021 Businesses outside that radius would not need a permit. Faimon Roberts, NOLA.com, "Loud music near homes sparks complaints in Jefferson Parish; council weighing options," 3 Feb. 2021

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

27 Feb 2021

Cite this Entry

“Radius.” Merriam-Webster.com Dictionary, Merriam-Webster, https://www.merriam-webster.com/dictionary/radius. Accessed 2 Mar. 2021.

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More Definitions for radius



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 radius (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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