Also called "RAS" (reliability, availability, serviceability) or "fault resilient," it refers to a multiprocessing system that can quickly recover from a failure. There may be a minute or two of downtime while one system switches over to another, but processing will continue. This is not the same as fault tolerant, in which redundant components are designed for continuous processing without skipping a heartbeat.
High availability also refers to being able to service a component in the system without shutting down the entire operation. See
clustering and
hot fix.