The Free On-line Dictionary of Computing (30 December 2018):

failover
failback

    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)