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.
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:
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.