A modification of any kind to a game, attempting to improve its look and feel, interface, gameplay elements, or balance. Questions about custom maps, custom game modes, custom UIs or even official but temporary modifications by the game developers themselves fall under this tag.
A mod, or modification, is the alteration of content from a video game in order to make it operate in a manner different from its original version. Mods can be created for any genre of game but are especially popular in first-person shooters, role-playing games 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 stand-alone software and require the user to have the original release in order to run.
They can include new items, modded weapons, characters, enemies, models, textures, levels, story lines, music, money, armor, life and game modes. 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 only are called unofficial patches.