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radius
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radius 		ra·di·us  audio  (rd-s) KEY 

NOUN:
pl. ra·di·i  (-d-) KEY  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.
  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.
ETYMOLOGY:
Latin, ray, spoke of a wheel, radius



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