A Dyson sphere is a massive structure completely enclosing a star; such a sphere would contain and absorb its star's total energy output, thus allowing for near-indefinite habitation of its interior. The concept was first theorized on Earth in the 1950s by Dr. Freeman Dyson.
The first Dyson sphere encountered by the Federation was discovered by the USS Enterprise-D in 2369, located in the Norpin sector. While studying the sphere, the Enterprise crew recovered Captain Montgomery Scott from the wreckage of the USS Jenolin, which crashed there in 2294. (TNG: "Relics")