The Wasteland is a term used to describe the United States after the nuclear apocalypse of 2077 in the Fallout Universe. Due to the dying plant life and the generally bleak appearance of the US, it was named a "wasteland".
Millions of people died during the 'Great War', and many more mutated into Ghouls or other things. Almost all of the water is irradiated in the Wasteland, except the rivers in the Capital Wasteland.
All of the US is considered a Wasteland, but there is specific parts of the Wasteland (i.e the Capital Wasteland).