High availability is a system design approach and associated service implementation that ensures a prearranged level of operational performance will be met during a contractual measurement period.
Users want their systems, for example wrist watches, hospitals, airplanes or computers, to be ready to serve them at all times. Availability refers to the ability of the user community to access the system, whether to submit new work, update or alter existing work, or collect the results of previous work. If a user cannot access the system, it is said to be unavailable. Generally, the term downtime is used to refer to periods when a system is unavailable.