Source : Free On-Line Dictionary of Computing

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)