- For other senses of this word, see radius (disambiguation).
In classical geometry, a radius of a circle or sphere is any line segment from its center to its boundary. By extension, the radius of a circle or sphere is the length of any such segment. The radius is half the diameter.
More generally — in geometry, engineering, graph theory, and many other contexts — the radius of something (e.g., a cylinder, a polygon, a graph, or a mechanical part) is the distance from its center or axis of symmetry to its outermost points. In this case, the radius may be more than half the diameter.
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