The Free On-line Dictionary of Computing (30 December 2018):

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)