A straight line from the center of a circle to any point on its circumference, or from the center of a sphere to any point on its surface, and the length of any such line. Also, a long, slightly curved bone, the shorter and thicker of the two forearm bones, located on the lateral side of the ulna. The plural form can be either radii or radiuses. Double the radius and you have the diameter.(pr. RAY-dee-əs)Also see radial, radiate, and ray.