In telecommunications, the round-trip delay time (RTD) or round-trip time (RTT) is the length of time it takes for a signal to be sent plus the length of time it takes for an acknowledgement of that signal to be received. This time delay includes the propagation times for the paths between the two communication endpoints.
In space technology, the round-trip delay time or round-trip light time is the time light (and hence any signal) takes to go to a space probe and return.
In the context of computer networks, the signal is generally a data packet, and the RTT is also known as the ping time. An internet user can determine the RTT by using the ping command.
End-to-end delay is the length of time it takes for a signal travel one direction and is often approximated by half the RTT.