A system-defined role is a role that you cannot edit or delete. The Administrator role is a system-defined role.
When you assign the Administrator role to a user or group for the domain, Analyst Service, Data Integration Service, Mass Ingestion Service, Metadata Manager Service, Model Repository Service, or PowerCenter Repository Service, the user or group is granted all privileges for the service. The Administrator role bypasses permission checking. Users with the Administrator role can access all objects managed by the service.