http://en.wikipedia.org/wiki/Online_and_offline

According to wiki,
"... online and offline are defined by Federal Standard 1037C."

and

"To be considered online, one of the following must apply to a device:
...
* 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)."

This is for telecommunications usage. Probably others have co-opted the term.