Source : Free On-Line Dictionary of Computing

out-of-band
     
        1.  The exchange of {call control} information
        on a dedicated channel, separate from that used by the
        telephone call or data transmission.
     
        2. 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
        {electronic mail}, such as telephone or {snail-mail}.
     
        4.  Values returned by a {function} that are not in
        its "natural" {range} of return values, but rather signal some
        kind of {exception}.  Many {C} functions that normally return
        a non-negative integer return -1 to indicate failure.
     
        This use confuses "out-of-band" with "out-of-range".  It is
        actually a clear example of {in-band} signalling since it uses
        the same "channel" for control and data.
     
        Compare {hidden flag}, {green bytes}, {fence}.
     
        [{Jargon File}]
     
        (2001-04-08)