- Date: circa 1611
- 1 : a line segment extending from the center of a circle or sphere to the circumference or bounding surface
- 2 a : the bone on the thumb side of the human forearm; also : a corresponding part of vertebrates above fishes
- b : the third and usually largest vein of an insect's wing
- 3 a : the length of a radius <a truck with a short turning radius>
In classical geometry, a radius of a circle or sphere is any line segment from its center to its perimeter. By extension, the radius of a circle or sphere is the length of any such segment, which is half the diameter.
More generally — in geometry, science, engineering, and many other contexts — the radius of something (e.g., a cylinder, a polygon, a mechanical part, a hole, or a galaxy) usually refers to the distance from its center or axis of symmetry to a point in the periphery: usually the point farthest from the center or axis (the outermost or maximum radius), or, sometimes, the closest point (the short or minimum radius). If the object does not have an obvious center, the term may refer to its circumradius, the radius of its circumscribed circle or circumscribed sphere. In either case, the radius may be more than half the diameter (which is usually defined as the maximum distance between any two points of the figure)