Most vendor marketing departments use the term to mean whatever they are trying to sell you. It's definitely not some level of nines, or a particular kind of technology.
SNIA offers a definition in their online dictionary:
"The ability of a system to perform its function continuously (without interruption) for a significantly longer period of time than the reliabilities of its individual components would suggest. High availability is most often achieved through failure tolerance. High availability is not an easily quantifiable term. Both the bounds of a system that is called highly available and the degree to which its availability is extraordinary must be clearly understood on a case-by-case basis."
It's not a bad definition, but it's still pretty amorphous.
My personal definition is as follows:
HA is a level of availability that is sufficient for the systems in question to provide the services for which they were implemented.
For life support systems or aeronautics systems, pretty much 100% is the rule; anything less and lives can be lost. But a system such as the one that fired the rockets on the original lunar landers (30+ years ago) only needed to fire once for just a few minutes and then never again. Their ability to function at any other time was totally irrelevant to their primary goal (to get astronauts off the moon).
So, whether or not your systems achieve HA depends on their availability requirements, and whether or not they achieve those requirements.
I'd actually take that a step further, and say that it's not system availability that matters, but application availability. After all, it's not the system that you use it's the application. The application uses the system, the storage, the network, other systems and so on. Each application has different business requirements.
Hope this is helpful.
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This was first published in December 2002