RTT - Round Trip Time. In network terms, the time it takes for a piece of data to get from the client to the server, plus the time it takes for the response to get back.

Also known as ping, or lag (when the RTT gets too high).

Most people play with RTTs below 200 milliseconds (0.2 seconds), but it's not uncommon for e.g. australian players to see RTTs well over 1 second to their servers in the US.