A system's ability to stay operational for as much time as possible, minimizing unplanned downtime. Measured as a percentage (e.g. 99.99% uptime).
In AZ-900 and CLF-C02 this is directly tied to provider SLAs. In SAA-C03 and AZ-104 it is implemented with availability zones, load balancers, and auto scaling. The "nines" (99.9%, 99.99%) are a common reference in questions.
High availability is a concept that can show up on the exam. Check the full guide to see it in context.