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In computing and especially computer networks, a lag is a symptom where result of an action appears later than expected. While different kinds of latency are well defined technical terms, lag is the symptom, not the cause.

Communications latency is the time taken for a packet of data to be sent from one application, travel to, and be received by another application. This includes transit time over the network, and processing time at the source and the destination computers. Specifically, this is the time for encoding the packet for transmission and transmitting it, the time for that serial data to traverse the network equipment between the nodes, and the time to get the data off the circuit. This is also known as "one-way latency". A minimum bound on latency is determined by the distance between communicating devices and the speed at which the signal propagates in the circuits (typically 70-95% of the speed of light). Actual latency is much higher, due to packet processing in networking equipment, and other traffic.

While strictly every packet experiences lag, the term lag is used to refer to delays noticeable to the user. Latency is directly related to the physical distance that data travels. Thus the time taken for a packet to travel from a computer server in Europe to a client in the same region is likely to be shorter than the time to travel from Europe to the Americas or Asia. But protocols and well written code that avoid unnecessary data transmissions are less affected by the latency inherent in a network. Modern corporate networks have devices to cache frequently requested data and accelerate protocols, thus reducing application response time, the cumulative effect of latency.

Inadequate network performance

Packets can be dropped or delayed by a congested or poorly designed network, or by distances that are too great for certain applications.

Inadequate server processing power

The server may have more work to do than it can handle. Examples include too many users to service, a world that's too complex to handle, or too many complex events from too many users at once.

Inadequate client computer processing power

The user's computer may not be fast enough to handle the processing load. This load includes handling the communications with the server, processing the user's input, and (significantly) drawing the scene. Very complex simulated worlds have a dramatic impact on client performance (usually measured as client frame rate). In addition, other programs and software can use computing power on the client machine.

Packet reordering and retries

Many communications protocols, especially those involved in on-line real-time gaming, require packets to be reassembled in order. If the network reorders the packets (which it is allowed to do), some protocol implementations drop the out-of-order packets and rely on packet retries. This causes delays in communications that is much greater than the network latency.

• Bandwidth - Measure of a connection's maximum data transfer capacity.
• Ping - Tool for determining network latency with regard to another system.
• Avalon - Movie by Mamoru Oshii, in which large parts of the portrayed society play an immense online virtual reality game, features lag as a phenomenon to which players react with bodily symptoms (convulsions, nausea).
• Lagometer - A 'device' that measures lag.
• Input lag
• Displacement Lag Another type of lag.

• The Gamer's Guide from Blues News - Lag Section
• It's the Latency, Stupid
• Bandwidth and Latency