As a concept 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, in order to enter or exit an enclave, one must cross 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.
The word enclave is sometimes used improperly to denote also a territory that is only partly surrounded by one or more other states. Small coastal territories that can be entered or exited by air or sea without crossing the territory of another state (e.g., Gibraltar, Ceuta, Monaco, Kaliningrad and Cabinda) are not actually enclaves. The expression "true enclave" is sometimes used to denote territories that strictly meet the definition of an enclave.
An exclave is defined as a portion of a country geographically separated from the main part by surrounding alien territory. An exclave is the enclave seen from the viewpoint of the main part. Thus, in figure at right, A3 is an enclave from the viewpoint of B but an exclave from the viewpoint of A, the main part. On the other hand, A2 is again an exclave of A, but is not an enclave, because it has boundaries with more than one entity. The word exclave is much less common than enclave, which tends to be the generic term to denote both concepts.