A data island is a data store

Data store

A data store is a data repository of a set of integrated objects. These objects are modeled using classes defined in database schemas. Data store includes not only data repositories like databases, it is a more general concept that includes also flat files that can store data.Some data stores do...

, such as on a PDA

Personal digital assistant

A personal digital assistant , also known as a palmtop computer, or personal data assistant, is a mobile device that functions as a personal information manager. Current PDAs often have the ability to connect to the Internet...

 or other computing device, that has non-existent or limited external connectivity. This limits the ability of the user to synchronize with or copy the data to other devices. Though new data can be added to the system, the ability to move that data elsewhere is impractical or impossible

Impossible

Impossible or Impossibility may refer to:-Law:* Impossibility, a valid legal excuse for non-performance of duties under a contract* Impossibility defense - a legal defense against criminal charges-Fictional characters:...

.Data islands, in general, contain a very huge set of data relative to its small physical space that it occupies.
The connectivity here does not necessarily imply a hardware interface. For example, it may be a result of poorly written system interface software.

A data island is a subset of entities that are connected to each other via relationships, but that are independent of other entities within the same data store.