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

inter-packet gap

    A time delay between successive data packets
   mandated by the network standard for protocol reasons.

   In Ethernet, the medium has to be "silent" (i.e., no data
   transfer) for a few microseconds before a node can consider
   the network idle and start to transmit.  This is necessary for
   fairness reasons.  The delay time, which approximately equals
   the signal propagation time on the cable, allows the "silence"
   to reach the far end so that all nodes consider the net idle.

   (1995-11-11)