The word radius (Latin for "wheel spoke"; plural radii, pronounced ray-dee-eye) has several meanings in English:

• In classical geometry, a radius of a circle or sphere is any line segment with one endpoint on the circle (i.e., the circular boundary) and the other at the center of the circle. By extension, the radius of a circle or sphere is the length of any such segment.

• More generally — in geometry, engineering, graph theory, and many other contexts — the radius of something (e.g. a cylinder, a graph, or a mechanical part) is the distance from its center or axis to its outermost points. See also diameter.

• In anatomy, the radius is one of the two bones of the forearm: see radius (bone).

• RADIUS is a remote access and dial-in protocol.
• Radius Fidonet mailer is an Argus successor.