In political geography, an enclave is a country or part of a country lying wholly within the boundaries of another country. Many entities are both enclaves and exclaves, but the two are not synonymous.

The word enclave crept into the jargon of diplomacy rather late in English, in 1868, coming from French, the lingua franca of diplomacy, with a sense inherited from late Latin inclavatus meaning “shut in, locked up" (with a key, late Latin clavis). The word exclave is a logical extension created three decades later.

Enclaves may be created for a variety of historical, political or geographical reasons. Some areas have been left as enclaves simply due to changes in the course of a river. Since living in an enclave can be very inconvenient and many agreements have to be found by both countries over mail addresses, power supply or passage rights, enclaves tend to be eliminated and many cases that existed before have now been removed.

Many coutries or parts of it are referred to as “enclaves” while this is not always the case. True enclves are those territories where a country is sovereign, but which cannot be reached without entering one particular other country. The best-known example was West Berlin, before the reunification of Germany, which was de facto a West German exclave within East Germany, and thus an East German enclave (many small West Berlin land areas, such as Steinstücken, were in turn separated from the main one, some by only a few meters). Some enclaves are countries in their own right, completely surrounded by another one, and therefore not exclaves (nor true enclaves), e.g. San Marino in Italy, Vatican City, Lesotho in South Africa.

In the world, there are more enclaves than states. Europe has, beneath Baarle-Hertog/Nassau, three other enclaves:

• Llivia is a Spanish enclave surrounded completely by French territory. It is situated in the French Pyrenees East of Andorra en West of Perpignan. It is separated from the rest of Spain by a 500 m wide gap of about 2 km.
• Büsingen am Hochrhein is a German town entirely surrounded by the Swiss canton of Schaffhausen and south across the Rhine by Zürich and Thurgau. It has a population of about 1,450 inhabitants. Since the late 18th century the exclave has been separated from the rest of Germany by a narrow strip of land which is less than 1km at its narrowest and contains the Swiss settlement of Dörflingen.
• Campione d'Italia is an Italian town and commune of Lombardy, occupying an enclave within the Swiss canton of Ticino, separated from the rest of Italy by Lake Lugano and mountains. The enclave has a gap of less than 1 km as the crow flies, but the rough terrain means the journey by road is over 10 km.

Nevertheless, Baarle-Hertog/Nassau forms the most complex enclave situation in the world, with its 30 enclaves and exclaves. Llivia, Büsingen en Campione each are one part of the puzzle, with one obvious way of access.