radius

American  

[rey-dee-uhs] / ˈreɪ di əs /

noun

plural

radii, radiuses

1. a straight line extending from the center of a circle or sphere to the circumference or surface.

   The radius of a circle is half the diameter.

2. the length of such a line.

3. any radial or radiating part.

4. a circular area having an extent determined by the length of the radius from a given or specified central point.

   every house within a radius of 50 miles.

5. a field or range of operation or influence.

6. extent of possible operation, travel, etc., as under a single supply of fuel.

   the flying radius of an airplane.

7. Anatomy. the bone of the forearm on the thumb side.

8. Zoology. a corresponding bone in the forelimb of other vertebrates.

9. Machinery Now Rare. the throw of an eccentric wheel or cam.

10. a rounded corner or edge on a machined or cast piece of metal.

11. Entomology. one of the principal longitudinal veins in the anterior portion of the wing of an insect.

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radius British  

/ ˈreɪdɪəs /

noun

1. a straight line joining the centre of a circle or sphere to any point on the circumference or surface

2. the length of this line, usually denoted by the symbol r

3. the distance from the centre of a regular polygon to a vertex ( long radius ) or the perpendicular distance to a side ( short radius )

4. anatomy the outer and slightly shorter of the two bones of the human forearm, extending from the elbow to the wrist

5. a corresponding bone in other vertebrates

6. any of the veins of an insect's wing

7. a group of ray florets, occurring in such plants as the daisy

8. 1. any radial or radiating part, such as a spoke

   2. ( as modifier )

      a radius arm

9. the lateral displacement of a cam or eccentric wheel

10. a circular area of a size indicated by the length of its radius

    the police stopped every lorry within a radius of four miles

11. the operational limit of a ship, aircraft, etc

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radius Scientific  

/ rā′dē-əs /

plural

radii

1. A line segment that joins the center of a circle or sphere with any point on the circumference of the circle or the surface of the sphere. It is half the length of the diameter.

2. The shorter and thicker of the two bones of the forearm or the lower portion of the foreleg.

3. See more at skeleton

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Etymology

Origin of radius

First recorded in 1590–1600; from Latin: “staff, rod, spoke, beam,” originally, “ray”; ray ¹

Example Sentences

In Modoc County, which borders Oregon and Nevada, one optometrist serves a 90-mile radius.

From Los Angeles Times • Apr. 4, 2026

By carefully adjusting the radius and depth of each cavity, they could control both the resonance wavelength and the vertical position of the optical mode.

From Science Daily • Mar. 24, 2026

It was the only thing within a multi-gate radius — a landscape of breakfast smash burgers and pepperoni pizza bagels — that didn’t make me queasy.

From Salon • Mar. 9, 2026

“Markets are panicked as investors fear that the blast radius of the Iran attack may be larger than initially suspected,” he wrote in a note Wednesday.

From Barron's • Mar. 5, 2026

If the stain were shaped like a circle, or like a square or a triangle, it would be easy to figure out; just take a ruler and measure its radius or its height and base.

From "Zero: The Biography of a Dangerous Idea" by Charles Seife