The fraction of time a hardware node is online and reachable, typically measured over a rolling 24-hour or 30-day window.
Most DePIN protocols tie reward multipliers to uptime: a node that is online 95% of the time earns more than one online 60% of the time. Some protocols (e.g. GEODNET) require a minimum uptime threshold before any rewards are issued. High uptime correlates with hardware reliability, stable power, and quality internet connectivity — all of which are operator responsibilities.