Mod

1. A modification / add-on that an individual or group of individuals create for a game. For example, there are many mods for such games as Battlefield, Quake, Unreal Tournament, Half-Life, etc. that enable a user to use different weapons, different vehicles, different character models, etc. The majority of all mods done today are freely distributed and available over the Internet.
2. A mod may also refer to a case mod, which are modifications to a computer that increase the speed of the computer and/or enhance its visual appearance. For example, a user may cut a window in the side of his or her case to make the inside of the computer visible. 

Also see:  Game definitions