In a Hot Standby System
The master database is the database in a hot standby system that is in the ONLINE operational state and with which the users are working. If the master database fails, one of the standby databases in the hot standby system automatically becomes the new master database.
Database Administration, Hot Standby System
Database Studio, Setting Up Hot Standby Systems: Overview
Database Administration in CCMS, Properties
Database Manager CLI, Managing Hot Standby Systems