Etymologically
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radius

This is the current revision of this entry, as edited by bogdan on 17 October 2026 at 16:19. This address is a permanent link to this version.
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Pronunciation

  • IPA: /ˈɹeɪ.di.əs/[1]

Etymology

It is borrowed from Latin radius (“staff, rod; spoke of a wheel; ray of light”),[2][3][4] a word of unknown origin,[3] perhaps related to rādīx (“root”), though de Vaan finds that unlikely.[3] It is a doublet of ray.[2]

Earliest recorded meaning

The staff of a cross-staff, a straight rod: 1597 in Barnhart and the OED, the 1590s in Etymonline;[2][5][3] Merriam-Webster dates a sense to 1578.[6]

Semantic evolution

As the shorter bone of the forearm it is from 1615 (the 1610s), as in Latin.[5][3][4] The geometric sense, a line from the centre of a circle to the circumference, is first recorded in Hobbes's Six Lessons (1656),[2][3] and a circular area of a given distance around a place from 1853.[3] The classical plural is radii.[3][7] The verb, to round off a corner, is from 1938.[5]

Timeline

  • 1578 – radius (Merriam-Webster).[6]
  • 1597 – the staff of a cross-staff.[2][5]
  • 1615 – the forearm bone.[5]
  • 1656 – the radius of a circle.[2]
  • 1853 – a circular area.[3]

Modern meaning

Noun

  1. A straight line from the centre of a circle or sphere to its edge, or its length.
  2. A circular area within a given distance of a place: within a ten-mile radius.
  3. (anatomy) The shorter, thicker of the two bones of the forearm.

Word family

  • circumradius
  • inradius
  • triradius
  • Bohr radius
  • hydraulic radius
  • radius of convergence
  • radius of curvature

References

  1. ^ Wiktionary, the free dictionary, https://en.wiktionary.org
  2. ^ a b c d e f Barnhart, Robert K. (1988), The Barnhart Dictionary of Etymology
  3. ^ a b c d e f g h i Harper, Douglas. Online Etymology Dictionary, https://www.etymonline.com
  4. ^ a b Hoad, T. F., ed. (1986), The Concise Oxford Dictionary of English Etymology, Oxford: Clarendon Press
  5. ^ a b c d e Simpson, J. A.; Weiner, E. S. C., eds. (1989), The Oxford English Dictionary, 2nd ed., Oxford: Clarendon Press
  6. ^ a b Merriam-Webster Unabridged Dictionary (2024), online ed., Springfield, Mass.: Merriam-Webster
  7. ^ Collins English Dictionary (2023), Glasgow: HarperCollins