Radii meaning : The long bone in the forearm, on the side of the thumb. The lighter bone (or fused portion of bone) in the forelimb of an animal. One of the major veins of the insect wing, between the subcosta and the media; the vein running along the costal edge of the discal cell. A line segment between any point of a circle or sphere and its center. The length of this line segment. Anything resembling a radius, such as the spoke of a wheel, the movable arm of a sextant, or one of the radiating lines of a spider's web.