Green Belt was implemented by the British Government in the 1950s as a way to check the spread of towns, and help villages retain their identity at a time of increasing housing and transport system develpoment. An area of green space was assigned around a town by the Local Planning Authority to act as a buffer between other towns. This was land that was not to be built on.
It even worked for a while
Unfortunately, due to a lack of scruples, large monetary incentives from big companies looking for green field sites and increased pressure on towns to provide new housing, 'Green Belt' is now a term used to decribe land that hasn't been built on yet.