1. <communications> The exchange of call control information on a dedicated channel, separate from that used by the telephone call or data transmission. 2. Sometimes used to describe what communications people call "shift characters", such as the ESC that leads control sequences for many terminals, or the level shift indicators in the old 5-bit Baudot codes. 3. In personal communication, using methods other than electronic mail, such as telephone or snail-mail. 4. <software> Values returned by a function that are not in its "natural" range of return values, but rather signal some kind of exception. Many C functions that normally return a non-negative integer return -1 to indicate failure. This use confuses "out-of-band" with "out-of-range". It is actually a clear example of in-band signalling since it uses the same "channel" for control and data. Compare hidden flag, green bytes, fence. [Jargon File] (2001-04-08)

Out-of-band activity is activity outside a defined telecommunications frequency band, or, metaphorically, outside of any primary communication channel. Protection from falsing is among its purposes.

In computer networking, out-of-band data is the data transferred through a stream that is independent from the main in-band data stream. An out-of-band data mechanism provides a conceptually independent channel, which allows any data sent via that mechanism to be kept separate from in-band data.