Definition of radius in English:


Syllabification: ra·di·us
Pronunciation: /ˈrādēəs

noun (plural radii /ˈrādēˌī/ or radiuses)

  • 1A straight line from the center to the circumference of a circle or sphere. See also illustration at geometric.
    More 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.
    More 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.
    More 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.
    More 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.
    More 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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