[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.