In international law, an enclave is any portion of a state that is entirely surrounded by the territory of another state. It follows from this definition that for an area to be considered an enclave, it must not be a sovereign state and it must not be entered or exited without the need to enter the territory of another state, either by land, sea or air. Enclaves, which were quite numerous in past centuries, are now very uncommon. The concept of enclave is applicable at both the international and sub-national level.
Somewhat abusively, the word enclave has progressively come into common usage to denote also any non-sovereign or sovereign territory, generally a small coastal territory, that is partly surrounded by one or several larger states. Thus, coastal territories such as Gibraltar, Ceuta, Monaco, Kaliningrad, Cabinda, etc., which can all be easily entered or exited by air or sea without the need to enter the territory of another state, are nevertheless called enclaves. The expression "true enclave" is often used to denote territories that correspond to the strict definition of an enclave.
An exclave is defined as a portion of a country geographically separated from the main part by surrounding alien territory. Basically, an exclave is the enclave seen from the viewpoint of the main part. Thus, in Fig. 1 at right, C is an enclave from the viewpoint of A but an exclave from the viewpoint of B, the main part. The word exclave is much less common than enclave, which tends to be the generic to denote both concepts.
In Fig. 2 at right, C is again an exclave of B, but is not an enclave, because it has boundaries with more than one entity.