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.