The radius is the distance from the center of a circle to its edge. It is also the name of one of the two long bones in your forearm, on the same side as your thumb.
It comes directly from Latin “radius,” meaning “ray, spoke of a wheel, or staff.” The geometric sense relates to the idea of a straight line spreading out from a center, like a spoke from the hub of a wheel.
In geometry, when you double the radius, you actually quadruple the area of the circle, which surprises many learners. The forearm bone called the radius got its name because it can rotate like the spoke of a wheel, letting your hand turn palm‑up and palm‑down.
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