Circles In classical geometry, a radius (plural: radii) 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. The radius is half the diameter. In science and engineering the term radius of curvature is commonly used as a synonym for radius. More generally—in geometry, engineering, graph theory, and many other contexts—the radius of something (for example, a cylinder, a polygon,… (More on Radius)