An entity defines a connection to the database, operating system (OS), or network that will be monitored.
Clicking on an entity in the Surveillance tree provides access to real-time views, flashback, recent history, rules, and stores.
Entities can be enabled or disabled, which controls whether rules and stores are started; disabling an entity will prevent alerts and historical data (but still allows real-time views, as well as recent history on data already collected). In a failover scenario, cluster failover actions can be defined in your clustering solution to automatically disable entities on the old server and enable entities (to the same databases) on the new server.
OS entities are added automatically to the Surveillance tree, but database entities must be created. In web client, you can create the entity using the most popular options, and there are notes provided on how to ensure that the database is configured properly. In the Windows client, you can use Entity Wizard, which will ensure that the entity can connect to the database instance and that it has the necessary permissions to monitor the database, as well as configure various monitoring options.