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Radius

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ra⋅di⋅us

  [rey-dee-uhs]
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–noun, plural -di⋅i  [-dee-ahy] , -di⋅us⋅es.
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. Compare ulna (def. 1).
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.
Origin:
1590–1600; < L: staff, rod, spoke, beam, orig., ray 1
Dictionary.com Unabridged
Based on the Random House Dictionary, © Random House, Inc. 2009.
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ra·di·us   (rā'dē-əs)   


(click for larger image in new window)
n.   pl. ra·di·i (-dē-ī') or ra·di·us·es

  1. Abbr. r or rad. Mathematics

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

    2. A line segment that joins the center of a sphere with any point on its surface.

    3. A line segment that joins the center of a regular polygon with any of its vertices.

    4. The length of any such line segment.

    5. A long, prismatic, slightly curved bone, the shorter and thicker of the two forearm bones, located on the lateral side of the ulna.

    6. A similar bone in many vertebrates.

  2. A circular area measured by a given radius: every family within a radius of 25 miles of the city center.

  3. A bounded range of effective activity or influence: the operating radius of a helicopter.

  4. A radial part or structure, such as a mechanically pivoted arm or the spoke of a wheel.

  5. Anatomy

    1. A long, prismatic, slightly curved bone, the shorter and thicker of the two forearm bones, located on the lateral side of the ulna.

    2. A similar bone in many vertebrates.


[Latin, ray, spoke of a wheel, radius.]
The American Heritage® Dictionary of the English Language, Fourth Edition
Copyright © 2009 by Houghton Mifflin Company.
Published by Houghton Mifflin Company. All rights reserved.
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Word Origin & History

radius 
1597, "cross-shaft," from L. radius "staff, spoke of a wheel, beam of light," of unknown origin. Perhaps related to radix "root," but Tucker suggests connection to Skt. vardhate "rises, makes grow," via root *neredh- "rise, out, extend forth;" or else Gk. ardis "sharp point." The geometric sense first recorded 1611. Plural is radii. Meaning "circular area of defined distance around some place" is attested from 1953. Meaning "shorter bone of the forearm" is from 1615 in Eng.; it was used thus by Roman writer Aulus Cornelius Celsus (1c.).
Online Etymology Dictionary, © 2001 Douglas Harper
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Medical Dictionary

Main Entry: ra·di·us
Pronunciation: 'rAd-E-&s
Function: noun
Inflected Form: plural ra·dii /-E-"I/ also ra·di·us·es
: the bone on the thumb side of the human forearm or on the corresponding part of the forelimb of vertebrates above fishes that in humans is movablyarticulated with the ulna at both ends so as to permit partial rotation about that bone, that bears on its inner aspect somewhat distal to the head a prominence for the insertion of the biceps tendon,and that has the lower end broadened for articulation with the proximal bones of the carpus so that rotation of the radius involves also that of the hand
Merriam-Webster's Medical Dictionary, © 2002 Merriam-Webster, Inc.
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Medical Dictionary

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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Computing Dictionary

RADIUS
Remote Authentication Dial-In User Service

The Free On-line Dictionary of Computing, © 1993-2007 Denis Howe
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Encyclopedia

radius

in anatomy, the outer of the two bones of the forearm when viewed with the palm facing forward. All land vertebrates have this bone. In humans it is shorter than the other bone of the forearm, the ulna.

Learn more about radius with a free trial on Britannica.com.

Encyclopedia Britannica, 2008. Encyclopedia Britannica Online.
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