Downtime (also called an outage) is any period when a system, network, or industrial equipment is unavailable, and it is the direct opposite of uptime. This unavailability typically stems from either unplanned events—such as system crashes, hardware malfunctions, or network communication failures—or from planned events like routine maintenance. In industrial and telecom settings, outages are formally classified by their start and end points, including "Turnkey" (begins when an operator stops the equipment, ends when normal production resumes), "Breaker to Breaker" (starts when the main power circuit is disconnected, ends when it is re-energized), and "Completion of Lock-out/Tag-out" (starts after all power sources are neutralized for safety, ending at mechanical completion). Interestingly, any online testing or performance tuning performed after the repair is not counted toward the official outage duration. Downtime is commonly measured over a shift or a 12-24 hour period, with failures often categorized by their operational, electrical, or mechanical origin, encompassing issues in hardware, software logic, interconnecting cables, or wireless transmission.