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.