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[rey-dee-uh s] /ˈreɪ di əs/
noun, plural radii
[rey-dee-ahy] /ˈreɪ diˌaɪ/ (Show IPA),
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.
the length of such a line.
any radial or radiating part.
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.
a field or range of operation or influence.
extent of possible operation, travel, etc., as under a single supply of fuel:
the flying radius of an airplane.
Anatomy. the bone of the forearm on the thumb side.
Compare ulna (def 1).
Zoology. a corresponding bone in the forelimb of other vertebrates.
Machinery Now Rare. the throw of an eccentric wheel or cam.
a rounded corner or edge on a machined or cast piece of metal.
Entomology. one of the principal longitudinal veins in the anterior portion of the wing of an insect.
Origin of radius
1590-1600; < Latin: staff, rod, spoke, beam, orig., ray1
Can be confused Unabridged
Based on the Random House Dictionary, © Random House, Inc. 2018.
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Examples from the Web for radius
Contemporary Examples
Historical Examples
  • Delve from the surface of your sphere to its heart, and at once your radius joins every other.

  • R represents the radius of the curve upon which the vehicle is moving.

  • If he kept her out of the radius of disapproval, she might never feel a shadow of regret.

    The Prisoner Alice Brown
  • It was then that Wilson stepped into the radius of shallow light.

    The Web of the Golden Spider

    Frederick Orin Bartlett
  • There isn't a well like mine in a radius of a hundred miles.

    The Forbidden Trail Honor Willsie
British Dictionary definitions for radius


noun (pl) -dii (-dɪˌaɪ), -diuses
a straight line joining the centre of a circle or sphere to any point on the circumference or surface
the length of this line, usually denoted by the symbol r
the distance from the centre of a regular polygon to a vertex (long radius) or the perpendicular distance to a side (short radius)
(anatomy) the outer and slightly shorter of the two bones of the human forearm, extending from the elbow to the wrist
a corresponding bone in other vertebrates
any of the veins of an insect's wing
a group of ray florets, occurring in such plants as the daisy
  1. any radial or radiating part, such as a spoke
  2. (as modifier): a radius arm
the lateral displacement of a cam or eccentric wheel
a circular area of a size indicated by the length of its radius: the police stopped every lorry within a radius of four miles
the operational limit of a ship, aircraft, etc
Word Origin
C16: from Latin: rod, ray, spoke
Collins English Dictionary - Complete & Unabridged 2012 Digital Edition
© William Collins Sons & Co. Ltd. 1979, 1986 © HarperCollins
Publishers 1998, 2000, 2003, 2005, 2006, 2007, 2009, 2012
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Word Origin and History for radius

1590s, "cross-shaft," from Latin radius "staff, stake, rod; spoke of a wheel; ray of light, beam of light; radius of a circle," of unknown origin. Perhaps related to radix "root," but Tucker suggests connection to Sanskrit vardhate "rises, makes grow," via root *neredh- "rise, out, extend forth;" or else Greek ardis "sharp point."

The geometric sense first recorded 1610s. Plural is radii. Meaning "circular area of defined distance around some place" is attested from 1953. Meaning "shorter bone of the forearm" is from 1610s in English (the Latin word had been used thus by the Romans).

Online Etymology Dictionary, © 2010 Douglas Harper
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radius in Medicine

radius ra·di·us (rā'dē-əs)
n. pl. ra·di·us·es or ra·di·i (-dē-ī')

  1. A line segment that joins the center of a circle with any point on its circumference.

  2. A long, prismatic, slightly curved bone, the shorter and thicker of the two forearm bones, located laterally to the ulna.

The American Heritage® Stedman's Medical Dictionary
Copyright © 2002, 2001, 1995 by Houghton Mifflin Company. Published by Houghton Mifflin Company.
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radius in Science
Plural radii (rā'dē-ī') or radiuses
  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. See more at skeleton.

The American Heritage® Science Dictionary
Copyright © 2002. Published by Houghton Mifflin. All rights reserved.
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