out-of-band

[from telecommunications and network theory]

1. In software, describes values of a function which are not in its `natural' range of return values, but are rather signals that some kind of exception has occurred. Many C functions, for example, return a nonnegative integral value, but indicate failure with an out-of-band return value of -1. Compare {hidden flag}, {green bytes}, {fence}.
2. Also 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 email, such as telephones or {snail-mail}.

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