The federated storage engine in MySQL is a storage engine that allows you to link multiple database tables, that exists in different instances of the database server, into logical databases. The federated storage engine was added to MySQL at version 5.0 and allows us to do various cool things like synchronous replication (using federated tables and triggers) and handle efficiently historical data.
You can read more about the federated storage engine in the Accessing Distributed Data with the Federated Storage Engine article by Robin Schumacher and in the the MySQL Reference Documentation.