from
The Free On-line Dictionary of Computing (8 July 2008)
failover
failback
<reliability> Automatically switching to a different,
{redundant} system upon {failure} or {abnormal termination} of
the currently active system. Failover can be applied to a
{cluster} of {servers}, to {network} or storage components or
any other set of redundant devices that must provide {high
availability} because down-time would be expensive or
inconvenient. It may be implemented in hardware, software or
a combination.
A "{hot standby}" is continuously active at the same time as
the failed system, using some kind of {load balancing} to
share the work, whereas a "{warm standby}" is ready to become
active at short notice.
When the failed system is operational again it may "failback",
i.e. become (one of) the active system(s) or it may become a
warm standby.
(2008-01-15)