Latency is the delay between an event and the reaction to it. It is usually desirable for latency to be low, though low latency usually comes at the cost of throughput, which is the rate at which events can be reacted to.
In networking, latency is usually expressed in terms of "ping time", measured using the ping(8) utility. Consistent and low latency is important for network games and voice chat (see VoIP) applications.
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