means The straight distance from the center of a circle or sphere to its edge — and, by extension, the range of reach around any central point.

from From Latin radius, meaning a rod, spoke, or ray of light — the same word the Romans used for the spokes of a wheel fanning out from the hub. That image of straight lines radiating from a center carried into geometry, and the bone of the forearm picked up the name too, for lying alongside the arm like a thin rod. The English word ray is a cousin from the same source.