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ra·di·us (rādē-əs)
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n. pl. ra·di·i (-dē-ī) or ra·di·us·es
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 in humans, the shorter and thicker of the two forearm bones, located on the lateral side of the ulna.
b. A similar bone in many other vertebrates.

[Latin, ray, spoke of a wheel, radius; see RAY1.]

The American Heritage® Dictionary of the English Language, Fifth Edition copyright ©2020 by Houghton Mifflin Harcourt Publishing Company. All rights reserved.
 

Indo-European & Semitic Roots Appendices

    Thousands of entries in the dictionary include etymologies that trace their origins back to reconstructed proto-languages. You can obtain more information about these forms in our online appendices:

    Indo-European Roots

    Semitic Roots

    The Indo-European appendix covers nearly half of the Indo-European roots that have left their mark on English words. A more complete treatment of Indo-European roots and the English words derived from them is available in our Dictionary of Indo-European Roots.

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