out-of-band

adj. [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.