An enclave is a territory (or a part of one) that is entirely surrounded by the territory of one other state.[1] Enclaves may also exist within territorial waters.[2]:60 Enclave is sometimes used improperly to denote a territory that is only partly surrounded by another state.[1] The Vatican City State and San Marino, both enclaved by Italy, and Lesotho, enclaved by South Africa, are completely enclaved sovereign states. An exclave is a portion of a state or territory geographically separated from the main part by surrounding alien territory (of one or more states).[3] Many exclaves are also enclaves, but are not necessarily so; an exclave can be surrounded by several states.[4] The Azerbaijani exclave of Nakhchivan is an example of an exclave that is not an enclave (bordering Armenia, Turkey and Iran).

Semi-enclaves and semi-exclaves are areas that, except for possessing an unsurrounded sea border (a coastline contiguous with international waters), would otherwise be enclaves or exclaves.[4]:116[5]:12–14 Enclaves and semi-enclaves can exist as independent states (Monaco, The Gambia and Brunei are semi-enclaves), while exclaves and semi-exclaves always constitute just a part of a sovereign state (like the Kaliningrad Oblast).[4]

A pene-exclave is a part of the territory of one country that can be conveniently approached—in particular, by wheeled traffic—only through the territory of another country.[6]:283 Pene-exclaves are also called functional exclaves or practical exclaves.[5]:31 Many pene-exclaves partially border their own territorial waters (i.e., they are not surrounded by other nations' territorial waters), such as Point Roberts, Washington and Minnesota's Northwest Angle. A pene-exclave can also exist entirely on land, such as when intervening mountains render a territory inaccessible from other parts of a country except through alien territory. A commonly cited example is the Kleinwalsertal, a valley part of Vorarlberg, Austria, that is accessible only from Germany to the north.

The Mauritania Railway. The inset shows the shorter route cutting through Western Sahara and the longer route within Mauritania through difficult terrain.