Greenfield Project

In software engineering jargon, a greenfield is a project which lacks any constraints imposed by prior work. The image is that of construction on greenfield land, where there is no need to remodel or demolish an existing structure. Such projects are often coveted by software engineers for this reason, but in practice, they can be quite rare.
Greenfield carries a similar meaning with modern wireless LAN networks. 802.11n wi-fi networks have an optional greenfield mode that improves efficiency by eliminating support for 802.11a/b/g devices.
Today, the term Greenfield is not restricted to software domain. Examples of greenfield projects are new factories, power plants, airports which are built from scratch on greenfield land. Those facilities which are modified/upgraded are called Brownfield projects (often the pre-existing site/facilities are contaminated/polluted.)
Greenfield also has meaning in sales. A greenfield opportunity refers to a marketplace that is completely untapped and free for the taking.