A mod may refer to any of the following:
1. A modification or add-on that an individual or group of individuals create for a game. For example, there are mods for such games as Battlefield, Quake, Unreal Tournament, Half-Life, Skyrim, etc. A mod may enable a user to use different weapons, different vehicles, different character models, or change the game entirely. 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 or enhance its visual appearance. For example, a user may cut a window in the side of their case with a CCFL inside to help show off the inside of the computer.
3. Mod is an abbreviation used for Modulo in programming.