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Concept
Online and offline
Summary
In computer technology and telecommunications, online indicates a state of connectivity and offline indicates a disconnected state. In modern terminology, this usually refers to an Internet connection, but (especially when expressed "on line" or "on the line") could refer to any piece of equipment or functional unit that is connected to a larger system. Being online means that the equipment or subsystem is connected, or that it is ready for use.
"Online" has come to describe activities performed on and data available on the Internet, for example: "online identity", "online predator", "online gambling", "online game", "online shopping", "online banking", and "online learning". Similar meaning is also given by the prefixes "cyber" and "e", as in the words "cyberspace", "cybercrime", "email", and "ecommerce". In contrast, "offline" can refer to either computing activities performed while disconnected from the Internet, or alternatives to Internet activities (such as shopping in brick-and-mortar stores). The term "offline" is sometimes used interchangeably with the acronym "IRL", meaning "in real life".
During the 19th century, the term on line was commonly used in both the railroad and telegraph industries. For railroads, a signal box would send messages down the line (track), via a telegraph line (cable), indicating the track's status: Train on line or Line clear. Telegraph linemen would refer to sending current through a line as direct on line or battery on line; or they may refer to a problem with the circuit as being on line, as opposed to the power source or end-point equipment.
Since at least 1950, in computing, the terms on-line and off-line have been used to refer to whether machines, including computers and peripheral devices, are connected or not. Here is an excerpt from the 1950 book High-Speed Computing Devices:
The use of automatic computing equipment for large-scale reduction of data will be strikingly successful only if means are provided for the automatic transcription of these data to a form suitable for automatic entry into the machine.
Official source
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