High availability (HA) is the idea that your application is available, meaning reachable, to your end users. In order to create highly available applications, your application code and the frontend that users interact with needs to be available the majority of the time. This term comes from the system design field, which defines the architecture, interface, data, and modules of a system in order to satisfy a given set of requirements. There are many examples of system design in disciplines from product development all the way to distributed systems theory. In HA, system design helps us understand the logical and physical design requirements to achieve a reliable and performant system.

In the industry, we refer to excellence in availability as five nines of availability. This 99.999 availability translates into specific amounts of downtime per day, week, month, and year. 

We can also look at the general availability formula, which you can use to understand a given system's availability:

Downtime per year in hours = (1 - Uptime Availability) x 365 x 24