In online gaming, lag is a noticeable delay between the action of players and the reaction of the server in a video game. Although lag may be caused by high latency, it may also occur due to insufficient processing power in the client (screen-lag). Screen-Lag can also happen in single player games as well.
Lag refers to slower response time (or latency), and to delays experienced in computing, communications, and engineering. (see also Lag (online gaming)).