In classical geometry, a radius of a circle or sphere's any line segment from its center to its perimeter. By extension, the radius of a circle or sphere's the length of any such segment, which's half the diameter. More generally — in geometry, science, engineering, and many other contexts — the radius of something (for example, a cylinder, a polygon, a mechanical part, or a galaxy) usually refers to the distance from its center or axis of symmetry to its outermost points. If the ob… (More on Radius)