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radius (n.)

1590s, "cross-shaft, straight rod or bar," from Latin radius "staff, stake, rod; spoke of a wheel; ray of light, beam of light; radius of a circle," a word of unknown origin. Perhaps related to radix "root," but de Vaan finds that "unlikely." The classical plural is radii

The geometric sense of "straight line drawn from the center of a circle to the circumference" is recorded from 1650s. Meaning "circular area of defined distance around some place" is attested from 1853. As the name of the shorter of the two bones of the forearm from 1610s in English (the Latin word had been used thus by the Romans).

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Definitions of radius from WordNet

radius (n.)
the length of a line segment between the center and circumference of a circle or sphere;
Synonyms: r
radius (n.)
a straight line from the center to the perimeter of a circle (or from the center to the surface of a sphere);
radius (n.)
a circular region whose area is indicated by the length of its radius;
they located it within a radius of 2 miles
radius (n.)
the outer and slightly shorter of the two bones of the human forearm;
radius (n.)
support consisting of a radial member of a wheel joining the hub to the rim;
Synonyms: spoke / wheel spoke
From wordnet.princeton.edu