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Merriam-Webster Online Dictionary
downtime (noun)
time during which production is stopped especially during setup for an operation or when making repairs
inactive time (as between periods of work) - napping during our downtime an injured athlete facing months of downtime
Merriam-Webster Online Thesaurus
downtime (noun)
a period of often involuntary inactivity or idleness
layoff, time-out, winter
break, breath, breather, interruption, lull, pause, recess; abeyance, doldrums, dormancy, latency, quiescence, suspense, suspension
Downtime (Wikipedia)

The term downtime is used to refer to periods when a system is unavailable. Downtime or outage duration refers to a period of time that a system fails to provide or perform its primary function. Reliability, availability, recovery, and unavailability are related concepts. The unavailability is the proportion of a time-span that a system is unavailable or offline. This is usually a result of the system failing to function because of an unplanned event, or because of routine maintenance (a planned event).

The term is commonly applied to networks and servers. The common reasons for unplanned outages are system failures (such as a crash) or communications failures (commonly known as network outage).

The term is also commonly applied in industrial environments in relation to failures in industrial production equipment. Some facilities measure the downtime incurred during a work shift, or during a 12- or 24-hour period. Another common practice is to identify each downtime event as having an operational, electrical or mechanical origin.

The opposite of downtime is uptime.

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