Source : Free On-Line Dictionary of Computing

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)