Final answer:
The 'master' database is the default System Database in SQL Server, containing essential system-level information for the instance.
Step-by-step explanation:
The default System Database in SQL Server is the master database. This database records all system-level information for an instance of SQL Server. This includes instance-wide metadata such as logon accounts, endpoint configurations, linked servers, and system configuration settings. In essence, the master database holds crucial information regarding SQL Server's operation, serving as a reference database for the entire SQL Server instance.