From HomoExcelsior

A shell built around a star to collect as much energy as possible, originally proposed by Freeman Dyson (although he admits to have borrowed the concept from Olaf Stapledon's novel Star Maker (1937)).

In the original proposal the shell consists of many independent solar collectors and habitats in separate orbits (also known as a Type I Dyson Sphere), but later people have discussed rigid shells consisting of only one piece (called a Type II Dyson Sphere). The latter construction is unfortunately both unstable (since it will experience no net attraction of the star), requires super-strong materials and have no internal gravity. The Dyson Sphere is a classic example of mega-technology and common in Science Fiction.

See also Ringworld.