The terms online and offline (also 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, such that even offline can be used in contrast to the common usage of online (e.g., "I bought that shirt offline").

Standard definitions

In computer technology and telecommunication, online and offline are defined by Federal Standard 1037C. They are states or conditions of a "device or equipment" or of "a functional unit". To be considered online, one of the following must apply to a device:

• Under the direct control of another device
• Under the direct control of the system with which it is associated
• Available for immediate use on demand by the system without human intervention
• Connected to a system, and is in operation
• Functional and ready for service

In contrast, a device that is offline meets none of these criteria (e.g., its main power source is disconnected or turned off, or it is off-power).

One example of a common use of these concepts is a Mail User Agent that can be instructed to be in either online or offline states. One such MUA is Microsoft Outlook. When online it will attempt to connect to mail servers (to check for new mail at regular intervals, for example), and when off-line it will not attempt to make any such connection. The online or offline state of the MUA does not necessarily reflect the connection status between the computer on which it is running and Internet. That is, the computer itself may be online—connected to Internet via a cable modem or other means—while Outlook is kept offline by the user, so that it makes no attempt to send or to receive messages. Similarly, a computer may be configured to employ a dial-up connection on demand (as when an application such as Outlook attempts to make connection to a server), but the user may not wish for Outlook to trigger that call whenever it is configured to check for mail.

Another example of the use of these concepts is in the world of digital audio technology. A tape recorder, digital editor, or other device that is online is one whose clock is under the control of the clock of a synchronization master device. When the sync master commences playback, the online device automatically synchronizes itself to the master and commences playing from the same point in the recording. A device that is offline uses no external clock reference and relies upon its own internal clock. When a large number of devices are connected to a sync master it is often convenient, if one wants to hear just the output of one single device, to take it offline because, if the device is played back online, all synchronized devices have to locate the playback point and wait for each other device to be in synchronization. (For related discussion, see MIDI timecode, word sync, and recording system synchronization.)