- radius
- 1597, "cross-shaft," from L. radius "staff, spoke of a wheel, beam of light," of unknown origin. Perhaps related to radix "root," but Tucker suggests connection to Skt. vardhate "rises, makes grow," via root *neredh- "rise, out, extend forth;" or else Gk. ardis "sharp point." The geometric sense first recorded 1611. Plural is radii. Meaning "circular area of defined distance around some place" is attested from 1953. Meaning "shorter bone of the forearm" is from 1615 in Eng.; it was used thus by Roman writer Aulus Cornelius Celsus (1c.).