RADIUS, properly a straight rod, bar or staff, the original meaning of the Latin word, to which also many of the various meanings seen in English were attached; it was thus applied to the spokes of a wheel, to the semi-diameter of a circle or Sphere and to a ray or beam of light, " ray " itself coming through the Fr. raie from radius. From this last sense comes " radiant," " radiation," and allied words. In mathematics, a radius is a straight line drawn from the centre to the circumference of a circle or to the surface of a Sphere; in anatomy the name is applied to the outer one of the two bones of the fore-arm in man or to the corresponding bone in the fore-leg of animals. It is also used in various other anatomical senses in botany, ichthyology, entomology, etc. A further application of the term is to an area the extent of which is marked by the length of the radius from the point which is taken as the centre; thus, in London, for the purpose of reckoning the fare of hackneycarriages, the radius is taken as extending four miles in any direction from Charing Cross.
Note - this article incorporates content from Encyclopaedia Britannica, Eleventh Edition, (1910-1911)