Source : Free On-Line Dictionary of Computing

bit stuffing
     
         A {protocol} which guarantees the receiver of
        {synchronous} data can recover the sender's clock.  When the
        data stream sent contains a large number of adjacent bits
        which cause no transition of the signal, the receiver cannot
        adjust its clock to maintain proper synchronised reception.
        To eliminate the possibility of such a pathological case, when
        a preset number of transitionless bits have been transmitted,
        a bit which does cause a transition is "stuffed" (transmitted)
        by the sender.  The receiver follows the same protocol and
        removes the stuffed bit after the specified number of
        transitionless bits, but can use the stuffed bit to recover
        the sender's clock.
     
        The advantage of bit stuffing is that only a bit (not a
        {byte}) is inserted in the data stream, and that only when the
        content of the data stream fails to provide a timing signal to
        the receiver.  Thus very nearly 100% of the bits transported
        are useful data.  In contrast, {asynchronous} transmission of
        data "throws away" a start bit and one or more stop bits for
        each data byte sent.
     
        (1996-04-23)