failover

noun1

1. •
   An automatic switch to a secondary system on failure of the primary system, such as a means for ensuring high availability of some critical resource (such as a computer system), involving a parallel backup system which is kept running at all times, so that, upon detected failure of the primary system, processing can be automatically shifted over to the backup.

   • Horizontal scaling gives you some failover and flexibility that you normally cannot get with only vertical scaling. — 2004