An empty, barren land that stood before the Black Gate of Mordor, devoid of all life, and filled only with ash and mud. Its only feature were its slag-mounds: conical piles of broken rock that marched on in row after row across the ashen landscape. Frodo and Sam came to the edge of the region on their travels to Orodruin, and later Gandalf led the armies of the Captains of the West to the same desolate region before facing the hosts of Mordor.
Notes
1
We know that the Desolation was created by Sauron's slaves, so it couldn't predate his arrival in Mordor. However, as we're never told specifically how the Desolation was created, it's hard to be certain when it first arose, and it's possible that it appeared at some point in the later history of Mordor.