An integration database is a database which acts as the data store for multiple applications, and thus integrates data across these applications (in contrast to an ApplicationDatabase).
An integration database needs a schema that takes all its client applications into account. The resulting schema is either more general, more complex or both. The database usually is controlled by a separate group to the applications and database changes are more complex because they have to be negotiated between the database group and the various applications.
The benefit of this is that sharing data between applications does not require an extra layer of integration services on the applications. Any changes to data made in a single application are made available to all applications at the time of database commit - thus keeping the applications' data use better synchronized.