Terra Incognita ("Unknown Land") is a Latin term applied to the lands beyond the extent of the international community.
By the late 21st century AD, the ancient world was a global international community of sovereign states, most of which were member states of the intergovernmental organization known as the United Nations. With the advent of Armageddon, the nuclear apocalypse of 2124 AD, most of the governments collapsed along with the UN. As vast regions of the Earth became uninhabitable, the human population was reduced to Europe and the Mediterranean Basin.
The modern world is considerably smaller than the world that existed before Armageddon and the subsequent Dark Ages. In the mid-31st century AD, the lands in western, central and eastern Europe were divided between multiple sovereign states. The rest of the Earth was simply referred to as "Terra Incognita," having long since been abandoned and virtually forgotten by the descendants of the humans who survived the nuclear war.