Green belt
A green belt or greenbelt is a policy and land use designation used in land use planning to retain areas of largely undeveloped, wild, or agricultural land surrounding or neighbouring urban areas. Similar concepts are greenways or green wedges which have a linear character and may run through an urban area instead of around it.  A green belt is basically an invisible line that goes around a certain area, stopping people from building there so that some of the wild and agricultural land can be saved.