Automatically switching to a backup system when the primary system fails. Like a backup generator kicking in instantly when the power goes out.
When the main database server crashed, failover automatically redirected all traffic to the backup server without users noticing.
Failover is a resilience pattern rather than a single product. DNS and load balancers can shift traffic to healthy endpoints, while managed databases and multi-zone/region designs provide automatic standby promotion or rerouting.