- radius (n.)
- 1590s, "cross-shaft," from Latin radius "staff, spoke of a wheel, beam of light," 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; it was used thus by Roman writer Aulus Cornelius Celsus (1c.).