MDB (message database)

An instance of a database implemented in Exchange server. A single MDB is normally identified as being public or private depending on the type of data that it stores. A single server running Exchange 2000 can accommodate up to 20 active MDBs. [Exchange 2000]

An instance of a database implemented in Exchange. A single MDB is identified as a mailbox store or public folder store, depending on the type of data that it stores. [Exchange 2003]