noun
radius
A line from a circle’s centre to its edge, or the distance represented by that line.
Where it comes from
Radius ordinarily means a line from a circle’s centre to its edge, or the distance represented by that line. English borrowed radius from Latin radius, a rod or staff that also meant a wheel's spoke, a ray of light, and a line from a circle's center to its edge. The Latin word's deeper origin is unknown. The same straight, outward-running image connects a spoke, a beam, and the geometrical line.
How the meaning developed
English later turned that line into an area: within a five-mile radius means everywhere no farther than that line's length from the center.
Sources
Definitions adapted from Open English WordNet where applicable. License and attribution. Word-history text comes from Lexifour’s sourced social research; pages are assembled automatically, not individually fact-checked on every update.
Updated 2026-10-05. Report a correction.