Display lag, also known as input lag, refers to the inherent latency between a console producing an output and a monitor screen displaying said output. This is different from frame delay, which is caused by the game itself intentionally delaying an input from being performed by a certain amount of time. Display lag is also separate from reaction time, which is mostly determined by the physical and cognitive ability of the player, though all three phenomena affect each other to influence the playing experience.
In most cases, it is caused by high-resolution monitors imposing additional processing on the visual data it receives on top of regular processing to analyze and produce every frame in order to make the image quality as high as possible. While this feature is useful for non-interactive applications, such as video playback and technical demos, it is often a hindrance when playing video games, as it goes against the real-time nature a majority of games require, thus resulting in a noticeable delay between input and action.
Display lag is difficult to quantify but easy for players to detect; controller feedbacks or inputs may be delayed, and the Wii U GamePad's screen may not be in sync with the television. It can be described as similar to playing online, though at a more constant and predictable rate.
Display lag is rarely considered a detriment with most video games, as the delay is usually so small that it is imperceivable execpt in extreme situations or if the player is actively searching for it. However, some types of games, such as rhythm games, fighting games and other games that require precise timing may become difficult or infeasible to succeed at even with small amounts of display lag. For the Super Smash Bros. series specifically, every frame of display lag decreases the possible reaction time of players, increasing the amount of prediction necessary to win and widening the size of a character's safe movepool.
Owing to concerns over display lag, tournaments for Smash 64, Melee, Brawl, and Project M heavily favour setups with older, cathode ray tube televisions (more commonly known simply as CRTs) that lack significant display lag. As Smash 4 and Ultimate do not properly support 4:3 screen ratios that CRT televisions use, modern televisions must be used for the games' tournaments. As a result, televisions used at such tournaments need to feature a "game mode" that reduces the effects of input lag, or need to be specifically marketed as a model intended for gaming.