When a system is unavailable for a certain period of time the term downtime is used to refer to that period. When a system fails to perform or provide its primary function called outage or downtime duration that period of time. Availability, unavailability, reliability, and recovery are related concepts. A system is unavailable or offline, the unavailability is the proportion of a certain time span. Because of routine maintenance or because of an unplanned event in the system is usually failing to function. System failures are the common reasons for unplanned downtime. In industrial environments, the term is also commonly used in relation to failures in industrial production equipment. To identify the downtime incurred during a work shift or during a 12 or 24 hour period. Another common practice is each downtime facilities measure events as having an operational, electrical, or mechanical origin. Uptime is the opposite of downtime.
The term outage is used in the industry. To avoid conflicts duration can have a different point of initiation and completion thus the following clarification should be used in contract execution.