High availability refers to a design methodology that is intended to keep a system available for use as much as is practicable. The intent is to minimize the causes of downtime such as system failures and maintenance, and to minimize the time it will take to detect and recover from downtime events that do occur. In practice, automated means of detecting and recovering from downtime events quickly become necessary as higher levels of availability must be achieved.
For more information about high availability, see the SUSE High Availability Guide.