A territory entirely surrounded by another country's territory. Used in legal and geographical contexts.
Vatican City is an enclave within Italy.
exclave
A part of a country separated from the main territory and surrounded by foreign land. Often contrasted with 飛び地 (とびち), a more common term for exclave.
Kaliningrad is a Russian exclave.
Compound of 飛び (とび, 'jumping' or 'detached') and 領土 (りょうど, 'territory'). The term is a modern legal coinage, likely influenced by the concept of 飛び地 (とびち).