Noun

radii

1. Plural form of radius.
   • 1767: Richard Helsham and Bryan Robinson, A Course of Lectures in Natural Philosophy, page 31

     Now the centrifugal forces of bodies revolving in the ſame time in different circles being to one another as the radii of the circles (as ſhall be ſhewn when I come to treat of thoſe forces) it follows that the centrifugal force muſt be greateſt at the æquator, and thence be continually diminished towards the poles.

Esperanto

Verb

radii (present tense radias, past tense radiis, future tense radios, conditional mood radius, jussive mood radiu)

1. to radiate

Latin

Noun

radii, radiorum m.; second declension

1. rays, plural of radius, ray

In classical geometry, a radius of a circle or sphere is any line segment from its center to its perimeter. By extension, the radius of a circle or sphere is the length of any such segment, which is half the diameter.

More generally — in geometry, science, engineering, and many other contexts — the radius of something (e.g., a cylinder, a polygon, a mechanical part, or a galaxy) usually refers to the distance from its center or axis of symmetry to its outermost points. If the object does not have an obvious center, the term may refer to its circumradius, the radius of its circumscribed circle or circumscribed sphere. In either case, the radius may be more than half the diameter (which is usually defined as the maximum distance between any two points of the figure).

The radius of a regular polygon (or polyhedron) is the distance from its center to any of its vertices; which is also its circumradius.

In graph theory, the radius of a graph is the minimum over all vertices u of the maximum distance from u to any other vertex of the graph.

The name comes from Latin radius, meaning "ray" but also the spoke of a chariot wheel. The plural in English is radii (as in Latin), but radiuses is also occasionally used.