I mean a part of a country that can't be reach from the rest without leaving the country or it's territorial water.
I don't know if exclave really covers what I'm thinking of. For example would the Azores be called an exclave of Portugal?
When we have landmass between the mainland and the rest, the only applicable terms are exclave and enclave.
For example, Kaliningrad is an exclave of Russia. Lesotho is an enclave of South Africa. Also, Uzbekistan has several exclaves inside Kyrgyzstan, which are enclaves for Kyrgyzstan itself. And several other examples.
But when there is water between the mainland and the rest the only term is overseas territory. Of course the distance should be very much so that the term applies.
For instance, we don't call Sicily an overseas territory of Italy! But Azores is an overseas territory of Portugal. Also, Guam is an overseas territory of the USA. And several other examples.
We don't have any terms for islands that are near the mainland; like Sicily, Isle of Man, etc.