radius

American  

[rey-dee-uhs] / ˈreɪ di əs /

noun

radii, plural radiuses plural

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 /

radii plural

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”; see ray ¹

Explanation

If you're a detective working a crime investigation, you might be told to "search over a one-mile radius," meaning, scope out everything that's within one mile of the crime scene. The radius of a circle is the distance from its center to the circumference, and if you are working on your geometry homework, or designing anything circular, you'll be writing down a little "r" quite frequently, to stand for "radius." It's also the name of one of the bones in your forearm.

Example Sentences

This information is sent within a radius of 50 nautical miles.

From Barron's • May 1, 2026

Geofence warrants instruct technology companies to turn over data about any of their users who passed through a given geographic radius during a particular window of time.

From The Wall Street Journal • Apr. 26, 2026

For the Sun, this radius is estimated to extend about 400 light years.

From Science Daily • Apr. 24, 2026

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

From Los Angeles Times • Apr. 4, 2026

The wolves outside the radius of Taisin’s power lifted their heads, looking in her direction.

From "Huntress" by Malinda Lo