An enormous ring-shaped habitat in space, in Iain Banks' Culture novels. An Orbital seems to be somewhat smaller than Larry Niven's Ringworld. In addition, an Orbital has a hub at its center (instead of a star like the Ringworld) where its Mind lives.
However, an Orbital must still has a circumference of millions of miles, and support several orders of magnitude more people than the Earth. All spinning so fast that the ring makes a complete rotation in a standard Culture day (a little bit more than 24 hours).
Orbitals are for those Culture citizens who prefer a more bucolic, relaxed life, as opposed to ships which scoot around doing exciting things, in addition to breeding more ships, as well as minds and crews for them.
(See A Few Notes On The Culture)