radius |ˈrādēəs|noun ( pl. radii |ˈrādēˌī| or radiuses)1 a straight line from the center to the circumference of a circle orsphere. radius 1See also illustration at geometric .• a radial line from the focus to any point of a curve.• the length of the radius of a circle or sphere.• a specified distance from a center in all directions : there are plenty of local pubs within a two-mile radius.2 Anatomy the thicker and shorter of the two bones in the human forearm. Compare with ulna .• Zoology the corresponding bone in a vertebrate's foreleg or a bird's wing.• Zoology (in an echinoderm or coelenterate) any of the primary axes of radial symmetry.• Entomology any of the main veins in an insect's wing.ORIGIN late 16th cent. (sense 2) : from Latin, literally ‘staff, spoke, ray.’