A Banks orbital is a ring megastructure orbiting a star, similar to a Bishop ring. Unlike a Ringworld or a Dyson Sphere, an orbital does not enclose the star. Like a ringworld, the orbital rotates to provide an analog of gravity on the inner surface. A Culture orbital rotates about once every 24 hours and has gravity-like effect about the same as the gravity of Earth, making the diameter of the ring about 3,000,000 km, and ensuring that the inhabitants experience night and day.
Along the rims of the inner surface are high walls that prevent water and air from escaping into space; these are foamed diamondoid to a height of hundreds of km. Solids, liquids, and gases comparable to those of the desired planetary type are held against the inner surface by centrifugal effects and prevented from slipping off the edge into space by these walls.
The tremendous size of a Banks orbital and the stresses involved mean that exotic matter is required for its construction, such as magmatter. They last for millions of years without maintenance before becoming uninhabitable.