A database is called replication master (database), when it is used as data source on a replication server. The replication master can be a replication database at the same time, when the master database is used in a replication bridge.
The replication master contains the most up-to-date version of data stored in the database. It contains also information about last updates and changes made by replication clients. Thus, the replication master can update replication databases also, when they had been off line for a longer period.