out-of-band: [from telecommunications and network theory] adj.

:out-of-band: [from telecommunications and network theory] adj.
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}.
-- The AI Hackers Dictionary