Definition - What does Uptime mean?
Uptime is a metric that represents the percentage of time that hardware, an IT system or device is successfully operational. It refers to when a system is working, versus downtime, which refers to when a system is not working.
Techopedia explains Uptime
One critical role of the terms uptime and downtime is defining the level of success provided by realtime services or systems. It can be difficult to quantify a service's success or value without these terms. A service level agreement (SLA) or other real-time service contract may include uptime/downtime ratios that show how much time a service is expected to remain operational.
In other cases, IT professionals use uptime to refer to a total consecutive amount of operational time. When a computer system has been consistently running for three weeks, one could refer to a "three-week uptime," or say, "uptime is at three weeks and counting."
In general, uptime is considered the default where uptime is contrasted with downtime. Downtime often requires specific explanation or description, such as downtime for maintenance, downtime as a result of malfunction/error or downtime resulting from crisis.