An Etymological Dictionary of Astronomy and Astrophysics

Of a circle, any straight line segment extending from the center to a point on the circumference.
Of a sphere, any straight line segment extending from the center to a point on the surface.
Of a regular polygon, the radius of the circle circumscribed about the polygon.

Etymology (EN): From L. radius “staff, spoke of a wheel, beam of light,” of unknown origin.

Etymology (PE): Šo’â’, loan from Ar.