noun, plural radii [rey-dee-ahy] , radiuses.
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 ). See diag. under skeleton.
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.

1590–1600; < Latin: staff, rod, spoke, beam, orig., ray1

circumference, diameter, radius, tangent.
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World English Dictionary
radius (ˈreɪdɪəs)
n , pl -dii, -diuses
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.  a.  any radial or radiating part, such as a spoke
 b.  (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
[C16: from Latin: rod, ray, spoke]

Collins English Dictionary - Complete & Unabridged 10th Edition
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Word Origin & History

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, © 2010 Douglas Harper
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American Heritage
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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American Heritage
Science Dictionary
radius   (rā'dē-əs)  Pronunciation Key 
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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Computing Dictionary

RADIUS definition

Remote Authentication Dial-In User Service
The Free On-line Dictionary of Computing, © Denis Howe 2010 http://foldoc.org
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Encyclopedia Britannica


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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Example sentences
As oxytocin comes into sharper focus, its social radius of action turns out to
  have definite limits.
The study's findings indicate cause for concern within a much larger radius
  than previously imagined around drill sites.
The construction begins with a circle of radius one.
That's not a big problem on its own, but the blast radius also seems to have
  been increased.
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