Definition of radius in English:

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Pronunciation: /ˈrādēəs/

noun (plural radii /ˈrādēˌī/ or radiuses)

Image of radius
1A straight line from the center to the circumference of a circle or sphere. See also illustration at geometric.
Example sentences
  • Thus, the area of a circle is equal to half of the product of the radius and the circumference.
  • He calculates the side of a regular pentagon in terms of the radius of the circumscribed circle.
  • Recall that given a circle of radius r, the circumference is 2pr.
1.1A radial line from the focus to any point of a curve.
Example sentences
  • These apply to clear zones on the outside of horizontal curves with a radius of 900 m or less.
  • Key factors to meet the new criteria include the length and width of runways, the width and curve radii of taxiways, and also the airport's pavement loading limits.
  • Second, this lightweight insert extends the bullet nose and accommodates use of a longer ogive - the radius of the curve of the bullet tip.
1.2The length of the radius of a circle or sphere.
Example sentences
  • The cone shaped tip is just under one micrometer in length and has a radius of a few nanometers at its apex.
  • Interestingly, though the analysis points to a ball with an average radius of 1.584 inches, the measured radius of a real baseball is 1.452 inches.
  • Stephenson, who constructed both projects, believed that gradients should be less than 1 percent and that curves should have very wide radii of at least a kilometre.
1.3A specified distance from a center in all directions: there are plenty of local pubs within a two-mile radius
More example sentences
  • It seems sensible to find somewhere with no population centres within a two-mile radius - like offshore.
  • The department has acquired legal powers to close all footpaths within a two-mile radius of any free-range poultry farms.
  • Teams divided into four groups, combing through a two-mile radius searching for any clue of what might have happened.
2 Anatomy The thicker and shorter of the two bones in the human forearm. Compare with ulna.
Example sentences
  • Common osteoporotic fracture sites include the vertebrae, the hip, the distal radius of the forearm, and the proximal humerus.
  • Scaphoid fractures are rare children and the elderly because of the relative weakness of the distal radius compared with the scaphoid in these age groups.
  • Abduction is movement of the hand away from the body as the proximal carpal bones move medially on the radius.
2.1 Zoology The corresponding bone in a vertebrate’s foreleg or a bird’s wing.
Example sentences
  • For example, the radius, one of the lower bones of the foreleg, became much broader.
  • As is usual in chelonioids, the radius is notably longer than the ulna.
  • In the equid foreleg, radius and ulna are united, and the ulna is greatly reduced so that all weight is born on the radius.
2.2 Zoology (In an echinoderm or coelenterate) any of the primary axes of radial symmetry.
2.3 Entomology Any of the main veins in an insect’s wing.


Late 16th century (sense 2): from Latin, literally 'staff, spoke, ray'.

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Syllabification: ra·di·us

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