In fault-tolerant systems, a failover is the function of a component that is taken over by a redundant component in the event of a fault. There are failover clusters that have high fault tolerance and can tolerate multiple faults simultaneously. These systems operate fault-tolerantly until no more redundant components are available to protect the system against further faults.
The term failover is very commonly applied to intelligent controllers that are all connected to the same storage unit and host. If a failure occurs in one of the controllers, then a functional controller takes over its function. In networking, failover functions can be taken over by bypasses or alternate routes. In the storage infrastructure, it is RAID systems that switch to other storage in the event of a fault.