radius

noun

ra·​di·​us ˈrā-dē-əs How to pronounce radius (audio)
plural radii ˈrā-dē-ˌī How to pronounce radius (audio) also radiuses
Synonyms of radiusnext
1
: a line segment extending from the center of a circle or sphere to the circumference or bounding surface
2
a
: 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
3
a
: 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.
Recent Examples on the Web
Examples are automatically compiled from online sources to show current usage. Read More Opinions expressed in the examples do not represent those of Merriam-Webster or its editors. Send us feedback.
Chasing cheap land and spare grid power, developers drop several campuses inside a 20-mile radius, which is what has happened around Abilene, Texas, and across swathes of Virginia. — Dara-Abasi Ita, Forbes.com, 20 July 2026 Authorities carried out the evacuations from buildings on slopes and in low-lying areas within a kilometer radius of the disaster site, relocating over 1,100 people, Chongqing authorities said in a WeChat post published Sunday. — ABC News, 19 July 2026 France’s Interior Minister Laurent Nuñez said experts found 10 points around a little more than half of a mile radius that were used for ignition, according to a report by France Insider. — Simon Constable, FOXNews.com, 19 July 2026 The helium signal extended well beyond the planet’s radius, which the researchers suggest is a sign that LHS 1140b has both a leading and a trailing tail of helium. — John Timmer, ArsTechnica, 16 July 2026 See All Example Sentences for radius

Word History

Etymology

Latin, ray, radius

First Known Use

1578, in the meaning defined at sense 1

Time Traveler
The first known use of radius was in 1578

Phrases Containing radius

Browse Nearby Words

radium vermilion
radius
radius bar
See all Nearby Words

Cite this Entry

“Radius.” Merriam-Webster.com Dictionary, Merriam-Webster, https://www.merriam-webster.com/dictionary/radius. Accessed 24 Jul. 2026.

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Kids Definition

radius

noun
ra·​di·​us ˈrād-ē-əs How to pronounce radius (audio)
plural radii -ē-ˌī How to pronounce radius (audio) also radiuses
1
: the bone on the thumb side of the human forearm
also : a corresponding bone of other vertebrates
2
: a line extending from the center of a circle or sphere to the circumference or surface
3
a
: the length of a radius
b
: a circular area defined by a given radius
deer may wander within a radius of several miles
4
: a radial part or plane
Etymology

from Latin radius "ray, beam, spoke" — related to radiate, radio, ray entry 2

Medical Definition

radius

noun
ra·​di·​us ˈrād-ē-əs How to pronounce radius (audio)
plural radii -ē-ˌī How to pronounce radius (audio) also radiuses
: 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

More from Merriam-Webster on radius

Thesaurus: All synonyms and antonyms for radius

Nglish: Translation of radius for Spanish Speakers

Britannica.com: Encyclopedia article about radius

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