Downtime refers to a period of time or a percentage of a timespan that a machine or system (usually a computer serverServer has several related meanings. In information technology, a server (also called a server application) is "an application program that accepts connections in order to service requests by sending back responses." (RFC 2616 (Hypertext Transfer Protocol ...server) is offlineThe terms on-line and off-line have specific meanings with respect to computer technology and telecommunication. The concepts have however been extended from their computing and telecommunication meanings into the area of human interaction and conversation.offline or not functioning, usually as a result of either system failure (such as a crashA crash in computing is a condition where a program (either an application or part of the operating system) stops performing its expected function and also stops responding to other parts of the system. Often the offending program may simply appear to freeze. If this program is a critical part of the operating system kernel the entire computer may crash (a system crash). Many ...crash) or routine maintenanceMaintenance, Repair and Operations or Maintenance, Repair and Overhaul (MRO), is fixing any sort of mechanical or electrical device should it become out of order or broken (repair) as well as performing the routine actions which keep the device in working ...maintenance. The opposite is uptimeUptime is a measure of the time a computer system has been "up" and running. It came into use to describe the opposite of downtime, times when a system was not operational. The uptime and reliability of computer and communications facilities is sometimes measured in nines. "Five nines" means 99.999% reliability, which translates to a total downtime of no longer than five minutes per year. It is often used as a measure of computer operating system reliability ...uptime.

• Maintenance downtimes have to be carefully scheduled in industries that rely on nearly 24-hour service, such as e-commerceElectronic Commerce is exactly analogous to a marketplace on the Internet. Electronic Commerce (also referred to as EC, e-commerce eCommerce or ecommerce) consists primarily of the buying and selling of products or services over electronic systems such as the Internet and other computer networks. The information technology ...e-commerce, medical informaticsHealth informatics or medical informatics is the intersection of information science, computer science and health care. It deals with the resources, devices and methods required to optimize the acquisition, storage, retrieval and use of information in health and biomedicine. Health informatics tools include not only computers but also clinical guidelines, formal medical ...medical informatics, news reportingA news agency is an organization of journalists established to supply news reports to organizations in the news trade: newspapers, magazines, and radio and television broadcasters. They are also known as wire services or news services. News agencies can be corporations that sell news (e.g. Reuters and All Headline News (AHN)), cooperatives composed of newspapers that share their articles with each other (e.g. AP), commercial newswire services which charge organizations to distribute their news (e.g. Market Wire, ...news reporting, and persistent online gamesA virtual world is a computer-based simulated environment intended for its users to inhabit and interact via avatars. This habitation usually is represented in the form of two or three-dimensional graphical representations of humanoids (or other graphical or text-based avatars). Some, but not all, virtual worlds allow for multiple users. The world being computer-simulated typically appears similar to the real world, with real world rules such as gravity, topography, ...persistent online games. In many cases, system-wide downtimes can be averted using rolling upgrades.
• Downtimes caused by system failures can have serious economic impact. To minimize these effects network monitoringThe term network monitoring describes the use of a system that constantly monitors a computer network for slow or failing systems and that notifies the network administrator in case of outages via email, pager or other alarms. It is a subset of the functions involved in network management. While an intrusion detection system monitors a network for threats from the outside, a network monitoring system monitors the network for problems due to overloaded ...network monitoring can be used.

In Service Level AgreementsService Level Agreement (SLA) is that part of a service contract where the level of service is formally defined. In practice, the term SLA is sometimes used incorrectly in the context of contracted delivery time (of the service) or performance.Service Level Agreements, it is common to mention a percentage value (per month or per year) that is calculated by dividing the sum of all downtimes timespans by the total time of a reference time span (e.g. a month). 0% downtime means that the server was available all the time.

For Internet servers downtimes above 1% per year or worse can be regarded as unacceptable as this means a downtime of more than 3 days per year. For e-commerce and other industrial use any value above 0.1% is usually considered unacceptable.