Greenbelt is an area of open land around a city, on which building is restricted.
See how Wiki defines Greenbelt -
A green belt or greenbelt is a policy and land use zone designation used in land use planning to retain areas of largely undeveloped, wild, or agricultural land surrounding or neighbouring urban areas. Wikipedia
Greenbelt is also used in Karate -
A green belt marking a level of proficiency in judo, karate, or other martial arts below that of a brown belt.
Hope it helps...