"Greenfield"

When reading job /contract ads I occasionally see the phrase "greenfield site". Does this mean that when you look out of the window you can see grass and trees?!?!?

Or is there a more subtle meaning?

I only ask because I have just seen an ad stating "greenfield site" with the location being London, City !!!

Greenfield means land which has never been built on.
Brownfield is the opposite - land which contains un-used/derelict buildings.

so, a woodland could be Greenfield site I supose.