Camera pickup's retention of an image after the camera has been moved, most common under low light levels. Comet tailing is a form of lag.
Input lag is used to describe the delay between an electrical input from the point of stimulation to action—for example, pressing a button and seeing the event after as little as 1/24th of a second.
In video games, input lag is either the delay between the television or monitor receiving a signal and it being displayed on the screen (see display lag below), or the delay between pressing a button and seeing the game react.
In electronic hardware development, input lag is the delay between an electronic input signal being generated (pressing a button as above) and processed (I/O ports have been read, and memory updated to reflect the state of the input). In this field, the phenomena detailed below, are referred to as output lag. More formally, the terms "input latency" and "output latency" are also used. Due to gamers being largely unaware of the phenomenon of input lag at this low level, the terms input lag and output lag became confused over time, and the above description, where input and output lag are combined into a singular phenomenon known as input lag, has become popular.
Overall, the correct terminology is clear: a delay between a physical input occurring (e.g., I/O pin voltage change) and it being processed electronically (I/O pins read by processor and memory registers updated to reflect the state of the pins) is input lag, and a delay between an electronic output being sent (e.g., memory register set to reflect the desired state of an output) and it being processed into a physically observable phenomenon (e.g., memory register read and I/O pin voltage modified accordingly), is output lag. In short, input lag occurs on input devices while output lag occurs on output devices.
The following are descriptions based on the colloquial use of the term, as used by gamers.