Lag, or sometimes also lagg (slang) means a phenomenon, that is most prominently felt during fast paced shooters but also in RTS games, when playing via network, particularly, the internet. As information is transmitted in packages over the network, the transmission of data may show a delay of one or more packages in comparison to normal round trip times from the client to the server and back. The quality of the connection can vary during a match, so lag can appear at any time, even when starting out with a nice connection. If the so called latency time spikes, it is usually noticable by the way objects move, including one's self. Instead of a straight smooth movement, the object jumps in a teleportation like fashion. This is especially annoying if the game relies heavily on tracking that object, like when aiming on an opponent player in a shooter. The probability of missing will be highly increased with no fault of the player himself.
This can also happen, if loss occurs (e. g. when playing via wireless LAN), and it may also happen right from the start, if the ping value (=latency time) is high from the start. Which value is acceptable for the game experience, depends strongly on the game you play. While a ping of 150 ms can still be quite playable for Star Trek: Voyager Elite Force, with Counter-Strike it leaves much to be desired.
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