High availability (HA) is simply the characteristic of keeping a system in an up state as much as possible, usually to some defined service level agreement such as 99.99% uptime (Less than 1 hour down a year).
To meet this HA requirement systems must:
- Be Fault tolerant - Capable of failing without loss of service
- Have Redundancy - Have hot swap at all times in event of failure
- Load balance - Spread access loads out to prevent overuse of a single point