1) The term greenfield was originally used in construction and development to reference land that has never been used (e.g. green or new), where there was no need to demolish or rebuild any existing structures. Today, the term greenfield project is used in many industries, including software development where it means to start a project without the need to consider any prior work.
Contrast with brownfield.
2) In wireless LAN (WLAN) technology, greenfield mode is a feature of major components of the 802.11n specification. The greenfield mode feature is designed to improve efficiency by eliminating support for 802.11a/b/g devices in an all draft-n network. In greenfield mode the network can be set to ignore all earlier standards.