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

software handshaking
XON/XOFF

    The transmission of extra data on a channel
   in order to control the device sending data in the other
   direction on that channel.  For an EIA-232 connection, this
   means sending Control-S and Control-Q characters to stop
   and start transmission.

   Since software handshaking requires the transmission and
   processing of extra data it can be less efficient than
   hardware handshaking.

   (1996-10-16)