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The plural of radius can be spelled either radii (as in the original Latin) or radiuses.
Definition of radius
noun (plural radii /ˈrādēˌī/ or radiuses)
- 1a straight line from the center to the circumference of a circle or sphere. See also illustration at geometric.
- 2 Anatomy the thicker and shorter of the two bones in the human forearm. Compare with ulna.
late 16th century (radius (sense 2)): from Latin, literally 'staff, spoke, ray'