Greenfield means a new piece of infrastructure ie. power plant, airport, gas pipeline, Telco, new train network etc which are built from scratch on what is essentially a green field, albeit it could be underwater ie. the JV of Transfield and Kumagai built the Sydney Harbour Tunnel almost 20 years ago.
See also Brownfield, Eleven Steps To Complete A Greenfield Or Brownfield Infrastructure Project Finance and Business Plan Developer.