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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Other Word Forms

Noun Inflected Forms

• radii
  plural
• radiuses
  plural

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

See Examples For:

radius radii radiuses

I was seemingly the only Asian child in a five-mile radius.

From Los Angeles Times ● Aug. 6, 2026

“The attack surface is exploding. Every ungoverned asset and identity is multiplying the blast radius of AI exponentially,” McDermott told MarketWatch.

From MarketWatch ● Jul. 22, 2026

Medical Center Barbour – the lone hospital in town that serves a roughly 60-mile radius across multiple counties – does not have an MRI machine.

From BBC ● Jul. 18, 2026

Bavi had been on track on Friday to be the largest typhoon to hit Taiwan in more than 30 years, but since then its strong-wind radius has shrunk to 350 kilometres, Cheng said.

From Barron's ● Jul. 11, 2026

"Forget about Cato, you're chasing off every rabbit in a ten-mile radius."

From "The Hunger Games" by Suzanne Collins

As the atomic numbers of these lanthanides increase, the radii of their ions decrease.

From Science Daily ● May 22, 2024

Overall, both mechanisms produce a lack of planets with sizes around two Earth radii.

From Science Daily ● Feb. 9, 2024

For comparison, the main part of Saturn's rings sits at just three planetary radii from the gas giant.

From Scientific American ● Feb. 11, 2023

In simulations, the researchers included an object about the size of Iapetus, Saturn’s third largest moon, orbiting about 43 Saturn radii out—between the orbits of Titan and Iapetus.

From Science Magazine ● Sep. 14, 2022

She turned back around, which took a moment because ponies do not have quick turning radii.

From "Chronicles of a Lizard Nobody" by Patrick Ness

"We come at night because of the congestion," Patton said, noting that each truck requires a two-driver team to handle the city's turning radiuses.

From BBC ● Jul. 12, 2026

Workers mapped out turning radiuses for buses, trimmed trees that would brush against bus windshields and painted dedicated bus lanes, she said.

From New York Times ● Aug. 23, 2022

The leg profile drove the design of the rest of the piece, making soft radiuses on all the surfaces that contact the body while keeping a clean, flush profile on the exterior of the chair.

From The Wall Street Journal ● Aug. 30, 2016

Importantly, they also do the boring and ore calculations based on different radiuses.

From Forbes ● Nov. 6, 2012

People have all kinds of words for them now but basically it was two radiuses mixed.

From Time Magazine Archive