failover

As a noun failover

is (computing|countable) 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.

As an adjective undefined is

lacking a definition or value.

As nouns the difference between failover and null

is that failover is (computing|countable) 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 while null is zero, nil; the cardinal number before einn.

As nouns the difference between fallback and failover

is that fallback is an act of falling back while failover is 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..

As an adjective fallback

is that can be resorted to as a fallback.

As nouns the difference between failover and failback

is that failover is 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. while failback is the restoration of a system in a state of failover back to its original state (before the failure occurred).