1. Of peripheral devices or files: not connected to the system or not usable. A device may be physically connected but offline if the system has been instructed not to use it.
2. Not connected to a network, especially the Internet.
Electronics and Electrical Engineering
Denoting a device, possibly an online device, that is switched off, broken, or disconnected from a computer. A device may be physically connected but offline if the computer system has been instructed not to use it. Compareonline.