radius

(rāˈdē-əs)

1. Abbr. r or rad. Mathematics
   a. A line segment that joins the center of a circle with any point on its circumference.
   b. A line segment that joins the center of a sphere with any point on its surface.
   c. A line segment that joins the center of a regular polygon with any of its vertices.
   d. 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
   a. A long, prismatic, slightly curved bone, the shorter and thicker of the two forearm bones, located on the lateral side of the ulna.
   b. A similar bone in many vertebrates.

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radius