type of communication on a communications medium that is sent immediately and directly and isn't held up by any non-out-of-band messages already in the pipeline
adj. Refers to a sentinel or otherwise exceptional value which is distinct from all normal values which can appear in some context (e.g. in a set of function return values, etc.). Compare in-band. Example: EOF (see question 12.1).
In telephony, information carried outside of the audio or voice channel.