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

Measured through the poles, the Earth's radius is 3,950 miles (6,357 km) — meaning the flyby was roughly 262 miles (423 km) above our planet. —Elizabeth Howell, Space.com, 3 Oct. 2025

Consequently, every knowledge worker must adopt a risk management mindset, constantly assessing the potential blast radius of their prompts and acting as a steward of their company’s data, controls, and compliance obligations. —Marco Argenti, Time, 2 Oct. 2025

The innermost planet is rocky, the second planet is likely a smaller, warmer version of Neptune, and the third exoplanet from its parent star, TOI 700d, is about 70% more massive and 19% larger in radius than Earth. —Ethan Siegel, Big Think, 2 Oct. 2025

There are several within a 30-minute radius, and here are a few of my favorites. —Jennifer Prince, Southern Living, 30 Sep. 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