1) Propagation delay, symbolized tpd, is the time required for a digital signal to travel from the input(s) of a logic gate to the output. It is measured in microseconds (µs), nanoseconds (ns), or picoseconds (ps), where 1 µs = 10-6 s, 1 ns = 10-9 s, and 1 ps = 10-12 s.
The propagation delay for an integrated circuit (IC) logic gate may differ for each of the inputs. If all other factors are held constant, the average propagation delay in a logic gate IC increases as the complexity of the internal circuitry increases. Some IC technologies have interently longer tpd values than others, and are considered "slower." Propagation delay is important because it has a direct effect on the speed at which a digital device, such as a computer, can operate. This is true of memory chips as well as microprocessors.
2) In a communications system, propagation delay refers to the time lag between the departure of a signal from the source and the arrival of the signal at the destination. This can range from a few nanoseconds or microseconds in local area networks (LANs) up to about 0.25 s in geostationary-satellite communications systems. Additional propagation delays can occur as a result of the time required for packets to make their way through land-based cables and nodes of the Internet.