noun pl. radii -·dii′ (-ī′) or radiuses -·uses

1. a raylike or radial part, as a spoke of a wheel
2. 1. any straight line extending from the center to the periphery of a circle or sphere
   2. the length of such a line
3. 1. the circular area or distance limited by the sweep of such a line no house within a radius of five miles
   2. the distance a ship or airplane can travel and still return to its point of origin without refueling
4. an extent, scope, range, etc. of a limited or specified kind within the radius of one's experience
5. 1. the shorter and thicker of the two bones of the forearm on the same side as the thumb
   2. a corresponding bone of the forelimb of a four-legged animal
6. Zool.
   1. any of the planes of division of the body of a radially symmetrical animal
   2. any of various longitudinal veins in the wing of an insect

Etymology: L, rod, spoke (of a wheel), hence radius, ray (of light), ? akin to radix, root