C-Prints 150 cm x 81 cm

Lag is a common word meaning to fail to keep up or to fall behind. In real-time applications, the term is used when the application fails to respond in a timely fashion to inputs.

In distributed applications (such as MMORPGs), lag is often caused by communication latency, which is the time taken for a sent packet of data to be received at the other end. It includes the time to encode the packet for transmission and transmit it, the time for that data to traverse the network equipment between the nodes, and the time to receive and decode the data. 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 in vacuum). Actual latency is often much higher because of packet processing in networking equipment, and other traffic.

The term lag is often also used as a synonym for communication latency.This can be misleading because there can be other causes for the symptom.