In general, when a machine is "online," it is turned on and connected to other devices. For example, when a network printer is online, computers connected to that network can print from it. Other devices, such as scanners, video cameras, audio interfaces, and others are said to be online when they are running and connected to a computer system.

Recently, however, the term "online" usually means being connected to the Internet. The connection can be through a phone line, using a dial-up or DSL modem, a cable line via a cable modem, or through a wireless connection. A computer can also be online via a connection to a computer network. Technically, computers that are on a network are online even if they are not connected to the Internet. But most networks are routed to a T1 line or other Internet connection anyway. When a computer or other device is not online, it is said to be offline.

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Definition from the PC Glossary