Final answer:
To define a server as the medium between an application and a data source in a Big SQL federation, the CREATE SERVER command should be used.
Step-by-step explanation:
To define a server that will serve as the medium between an application and a data source in a Big SQL federation, you should use the CREATE SERVER command. This command is specifically designed to create a server object in the SQL federation that represents a data source. It allows the federation to communicate and exchange data across different databases and external data sources. By defining a server in this way, you enable your database to query and process data from diverse sources as if it were all part of a single, unified database.