radius (English)

Where does "radius" come from?

The English word 'radius' is a direct loan from Late Latin, where it originally referred to a rod, staff, or a spoke of a wheel. While its exact pre-Latin origins remain uncertain, linguists have linked it to ancient roots suggesting a beam, post, or even the sharp point of an arrow. Over time, the term evolved to describe the geometric line segment extending from the center of a circle, which also makes it a doublet of the word 'ray'.

Full etymology tree

• radius (English), 'The long bone in the forearm, on the side of the thumb.'
  • borrowed radius (Latin), 'a staff, rod'
    • influence ἄρδις (árdís) (Ancient Greek), 'point of an arrow'
      • inherited *h₂erdi- (Proto-Indo-European)
    • inherited *reh₁t- (Proto-Indo-European), 'post, beam, pole'
      • suffix *Hreh₁- (Proto-Indo-European)
    • inherited *h₃er- (Proto-Indo-European), 'to move, to stir'
    • inherited *neredʰ- (Proto-Indo-European)

Direct ancestors

• radius (Latin)
• ray (English)

Updated June 2026

Etymology data from Wiktionary, CC BY-SA 4.0 · About Miraris