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

hardware handshaking

    A technique for regulating the flow of data
   across an interface by means of signals carried on separate
   wires.

   A common example is the RTS (Request to Send) and CTS (Clear
   to Send) signals on an EIA-232 serial line.

   The alternative, software handshaking, uses two special
   characters inserted into the data stream to carry the same
   information.

   (1995-01-23)