Mod or modification is a term generally applied to computer games, especially first-person shooters and real-time strategy games. Mods are made by the general public, and can be entirely new games in themselves. They can include new items, weapons, characters, enemies, models, modes, textures, levels, story lines and game modes. They also usually take place in unique locations. They can be single-player or multiplayer. Mods that add new content to the underlying game are often called partial conversions, while mods that create an entirely new game are called total conversions.
Games running on a PC are often designed with change in mind, and this consequently allows modern computer games to be modified by gamers without much difficulty. These mods can add extra replay value and interest. The Internet provides an inexpensive medium to promote and distribute mods, and they have become an increasingly important factor in the commercial success of some games. Developers such as id Software, Valve Software, Bethesda Softworks, Relic Entertainment and Epic Games provide extensive tools and documentation to assist mod makers, leveraging the potential success brought in by a popular mod like Counter-Strike.
Mods can significantly outshine or continue the success of the original game. Playing a mod might even become more common than playing the unmodified original. In those cases, players might have to clarify that they are referring to the unmodified game when talking about playing a game. In some cases the term vanilla is used make this distinction, "vanilla Battlefield 1942", for example, refers to the original, unmodified game. For vanilla games, prefix "v" or "V" is commonly used together with acronymed game title, eg. VQ3 stands for "vanilla Quake 3".
It should also be noted that while games such as Doom and Quake are not often thought of when concerning game modification they have nonetheless been the pioneering and pinnacle reason for the modern day mod scene. One particular mod, Counter-Strike, originated from a Quake mod called Navy Seals featuring real-world weapons and headshots, also from the same developer.
Recently, computer games have also been used as a digital-art medium. See artistic computer game modification.