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

flow control

    The collection of techniques used
   in serial communications to stop the sender sending data until
   the receiver can accept it.  This may be either software flow
   control or hardware flow control.  The receiver typically
   has a fixed size buffer into which received data is written
   as soon as it is received.  When the amount of buffered data
   exceeds a "high water mark", the receiver will signal to the
   transmitter to stop transmitting until the process reading the
   data has read sufficient data from the buffer that it has
   reached its "low water mark", at which point the receiver
   signals to the transmitter to resume transmission.

   (1995-03-22)