1590s, "cross-shaft," from Latin radius "staff, stake, rod; spoke of a wheel; ray of light, beam of light; radius of a circle," of unknown origin. Perhaps related to radix "root," but Tucker suggests connection to Sanskrit vardhate "rises, makes grow," via root *neredh- "rise, out, extend forth;" or else Greek ardis "sharp point."
The geometric sense first recorded 1610s. Plural is radii. Meaning "circular area of defined distance around some place" is attested from 1953. Meaning "shorter bone of the forearm" is from 1610s in English (the Latin word had been used thus by the Romans).
radius ra·di·us (rā'dē-əs)
n. pl. ra·di·us·es or ra·di·i (-dē-ī')
A line segment that joins the center of a circle with any point on its circumference.
A long, prismatic, slightly curved bone, the shorter and thicker of the two forearm bones, located laterally to the ulna.
Plural radii (rā'dē-ī') or radiuses