1. Pertaining to the operation of a functional unit when not under the direct control of the system with which it is associated.
Note 1: Off-line units are not available for immediate use on demand by the system.
Note 2: Off-line units may be independently operated.
See also: offline browsing.
2. Pertaining to equipment that is disconnected from a system, is not in operation, and usually has its main power source disconnected or turned off (off-power).
3. Being offline, is the state of being away from a computer that is connected to the Internet. People who are used to being online all day long can suffer from withdrawal symptoms if they are offline for extended periods.
Source: from Federal Standard 1037C and from MIL-STD-188