a straight line from the center to the perimeter of a circle (or from the center to the surface of a sphere)
"The radius extends as a straight line from the exact center of the wheel to its outer rim."
a circular region whose area is indicated by the length of its radius
"they located it within a radius of 2 miles"
the outer and slightly shorter of the two bones of the human forearm
"The radius rotates within the ulna to allow me to turn my palm upward while holding a cup."
support consisting of a radial member of a wheel joining the hub to the rim
"The mechanic tightened the loose radius on the bicycle wheel to prevent further wobbling."
The long bone in the forearm, on the side of the thumb.
"The server administrator configured the RADIUS protocol to manage secure access for all remote employees."
Initialism of Remote Authentication Dial-In User Service.
In plain English: The radius is the straight-line distance from the center of a circle to any point on its edge.
"The city's parks are located within a five-mile radius of downtown."
Usage: Radius refers specifically to the thinner bone located between your wrist and elbow that is positioned next to the thumb when you hold out your hand. Do not confuse this anatomical term with "radius" as a measure of distance from a center point; context will clarify which meaning applies in any given sentence.
The word radius comes directly from Latin, where it originally meant a "ray." It entered English as a doublet alongside the word ray, sharing that same fundamental meaning but developing distinct uses over time.