The ability of an IT service or other configuration item to perform its agreed function when required. Availability is usually calculated as a percentage. It is often measured by the mean time between failures (MTBF) and mean time to restore service (MTRS).
Downtime costs money and erodes trust. In high-frequency transactional environments, even 99.9% availability (approx. 9 hours of downtime/year) may be unacceptable. Poor availability management is the fastest way to lose customers.
Availability is not luck; it is a feature you design for. We engineer for failure, assuming components will crash. By implementing self-healing systems (like Kubernetes auto-scaling) and redundant architectures (multi-region active-active), we make availability a property of the system, not a hope.