For the modern North American meaning as a type of housing, see Condominium.
In international law, a condominium (plural either condominia, as in Latin, or condominiums) is a political territory (state or border area) in or over which two or more sovereign powers formally agree to share equally dominium (in the sense of sovereignty) and exercise their rights jointly, without dividing it up into 'national' zones.
Although a condominium has always been recognized as a theoretical possibility, condominiums have been rare in practice. A major problem, and the reason why so few have existed in practice, is the difficulty of ensuring co-operation between the sovereign powers; once the understanding fails, the status is likely to become untenable.
The word is recorded in English since c.1714, from Modern Latin, apparently coined in Germany c.1700 from Latin com- "together" + dominium "right of ownership" (compare domain).
The Moselle River, is a condominium between Luxembourg and Germany, who share bridges and at least the tip of one island (near Schengen). Established by Treaty in 1816.
Pheasant Island (also known as Conference Island, Konpantzia in Basque, Île de la Conférence in French or Isla de los Faisanes in Spanish) in the River Bidassoa between France and Spain. Established by the Treaty of the Pyrenees in 1659.
The main part of Lake Constance (without islands) is considered by Austria to be a condominium between Germany, Austria and Switzerland. This view is not shared by Germany and Switzerland.
The term is sometimes even applied to a similar arrangement between members of a Monarch's countries in (personal or formal) union, as was the case for the district of Fiume (Rijeka), shared between Hungary and Croatia within the Hapsburg Empire since 1868.
Co-principality
Under French law, Andorra was once considered to be a French–Spanish condominium, although it is more commonly classed as a co-principality, since it is itself a sovereign state, not a possession of one or more foreign powers, even though the quality of Head of State is shared ex officio by two foreigners.
Erfurt, from 12th century until Thirty Years' War shared between Archbishopric of Mainz and Counts of Gleichen, the latter replaced by the city councel in 1289 (Concordata Gebhardi), Landgrave of Thuringia in 1327 and the House of Wettin in 1483 (Treaty of Weimar)
Sakhalin Island in the Far East, off Siberia's Pacific coast. In 1855, the Russian and Japanese empires signed the Treaty of Shimoda, allowing both countries' nationals to inhabit the island: Russians in the north, and Japanese in the south, without a clear boundary in between. After the Russo-Japanese War ended in 1905 Japan annexed the territory south of 60 North Latitude. In 1945, according to Yalta Conference agreements, the Soviet Union took over the control of the whole island.
New Hebrides formed a French–British condominium in 1906 until independence in 1980 as a republic, now called Vanuatu
Nauru a tripartite condominium mandate territory administered by Australia, New Zealand and United Kingdom from 1923 to 1942 and again in 1947 as a trust territory until independence in 1968
Samoan Islands from 1889 to 1899 were a rare tripartite condominium under joint protectorate of Germany, Britain and the USA
Anglo-Egyptian Sudan was a British–Egyptian condominium until 1956; however, this was largely meaningless as Egypt was a de facto British colony through 1952 and British troops only left Egypt in 1956