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This article explains the word Mod. (source wikipedia.org)

Mod or modification is a term generally applied to PC games, especially first-person shooters, RPGs and real-time strategy games. Mods are made by the general public or a developer, and can be entirely new games in themselves, but mods are not standalone software and require the user to have the original release in order to run. They can include new items, weapons, characters, enemies, models, textures, levels, story lines, music, 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 and mods that fix bugs are called unofficial patches.

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, Firaxis, Crytek, The Creative Assembly,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 and/or continue the success of the original game even when it is dated. 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".

As early as the 1980s, computer game mods have also been used for the sole purpose of creating art, as opposed to an actual game. This can include recording in-game footage into a movie, as well as attempting to reproduce real-life areas inside a game with no regard for gameplay value. See artistic computer game modification, machinima and demoscene.