The 1933 Montevideo Convention listed a few necessary categories a nation needed to have in order to be considered a nation. These categories included having a permanent population, having a defined piece of land or territory this population resides on, having some sort of governmental structure and, finally, having the ability to enter into relations with other states. This last category causes some problems for so-called ghost states, which are unrecognized by the majority of nations currently existing on Earth.
Ghost states are states that function just like any other nation, but for various political reasons go unrecognized by the rest of the international community. Such ghost states today include the Saharawi Arab Democratic Republic, which is located in the country of Western Sahara. Morocco technically claims ownership of the land but the group of people living in Western Sahara have the right to self-determination. They currently exist in a semi-accepted state of being, with some nations accepting their existence and others not.
Many ghost states are comprised of ethnic minority groups who feel at odds with the majority of the nation around them. These groups have occasionally gone to war, fighting to become their own national entity in the grand scheme of international politics. Some ghost states that fall under this category include South Ossetia, which declared its independence from Georgia in 1990 and was later invaded by Russia in 2008. South Ossetia is only recognized by Russia, Nicaragua, Venezuela, and Nauru. These same four nations also endorse the nation of Abkhazia, another remnant of the disintegration of the USSR.
There are a few ghost states that no one recognizes, including the Islamic State (ISIS). Two slightly more functional ghost states are Transnistria, part of Moldova, and the Nagorno-Karabarkh Republic, which is part of Azerbaijan. These two nations are considered autonomous and have their own currencies, governments, and functioning social systems.