A system's ability to keep operating correctly when one or more of its components fail. It implies active redundancy, where the system keeps running without interruption.
AZ-900 and CLF-C02 distinguish this from high availability: fault tolerance means zero downtime even when failures occur, while high availability may allow a brief interruption. More expensive to implement.
Fault tolerance is a concept that can show up on the exam. Check the full guide to see it in context.