[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 either a nonnegative integral value, or indicate failure with an out-of-band return value of -1.
2. Also sometimes used to describe what communications people call 'shift characters', like 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.