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.
L’Oréal, the parent company of Redken and Kérastase, developed a curl classification system from a study across 22 countries, studying curl radius, shrinkage ratio, and the number of twists on a hair fiber. — Ava Welsing-Kitcher, Allure, 24 Aug. 2026 Within a six-inch radius, chances are that Pedicularis has tapped into the roots of Cassiope, drawing resources vital to the plant’s growth and survival. — Denise Hruby, Miami Herald, 21 Aug. 2026 Everyone within a 15 mile radius works for Aprocetu; family bonds bind it together. — Jennifer Leigh Parker, Forbes.com, 21 Aug. 2026 According to a letter from a Meridian long-range planner included in the project’s application materials, there have been 1,988 single-family homes approved within a 1-mile radius of Summerlin in the past five years. — Rose Evans august 20, Idaho Statesman, 20 Aug. 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 28 Aug. 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

Britannica.com: Encyclopedia article about radius

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