radius

noun

ra·​di·​us ˈrā-dē-əs 

plural radii ˈrā-dē-ˌī  also radiuses

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

Other highlights include a 24/7 fitness center with downtown views, a culinary concierge that can help arrange dining and cultural experiences around town, free cruiser bikes for wheeling around, and a complimentary house car that can take you anywhere within a five-mile radius. —Mae Hamilton, Travel + Leisure, 24 May 2025

In a separate statement on its website, OpenAI said Stargate UAE has the potential to provide AI infrastructure and computing capacity within a 2,000-mile radius, reaching up to half the world’s population. —Mostafa Salem, CNN Money, 22 May 2025

Resort guests can hop on complimentary bikes and follow a mapped route highlighting Islamorada’s most delicious key lime — inspired bites and sips, all within an easy three-mile radius of the property. —Angela Caraway-Carlton, Miami Herald, 20 May 2025

The blast at American Reproductive Centers, which the FBI deemed an act of terrorism, caused damage to multiple buildings and businesses within a 250-yard radius. —Elizabeth Robinson, NBC news, 19 May 2025

Word History

Etymology

Latin, ray, radius

First Known Use

1578, in the meaning defined at sense 1

Phrases Containing radius

• radius of curvature
• radius vector

Kids Definition

radius

noun

ra·​di·​us ˈrād-ē-əs 

plural radii -ē-ˌī  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 

plural radii -ē-ˌī  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