A radius is a straight line from the center of a circle to any point on its edge. If you draw a circle with a compass, the radius is the distance from the sharp point in the middle to the pencil marking the circle's edge.

Every circle has countless radii (that's the plural of radius), but they're all exactly the same length. That's what makes a circle perfectly round: every point on the edge sits exactly the same distance from the center. When you know a circle's radius, you can figure out important things about it, like how far around it is (the circumference) or how much space it covers (the area).

The word also describes a surrounding area measured from a central point. If someone says “search within a five-mile radius,” they mean everywhere that's five miles or less from a starting point in any direction, forming an invisible circle. A pizza delivery place might serve customers within a three-mile radius of its restaurant.

One of the bones in your forearm is also called the radius. When you rotate your hand, that bone moves in a circular path around the other forearm bone, like a radius spinning around a center point.