Greenfield land is undeveloped land in a city or rural area either used for agriculture, landscape design, or left to evolve naturally. These areas of land are usually agricultural or amenity properties being considered for urban development.
Greenfield land can be unfenced open fields, urban lots or restricted closed properties, kept off limits to the general public by a private or government entity.
Rather than building upon greenfield land, a developer may choose to redevelop brownfield or greyfield lands, areas that have been developed but left abandoned or underused.(Wikipedia)