Mod

A mod&mdash;or modification&mdash;is a collection of data and/or software which as the name suggests, modifies a game.

The popularity of mods in non-Free software might seem a priori to be influenced by the fact that the players typically are not able to alter the base game in the way they might want to, which remains a possibility with Free games. Despite this mods remain popular with Free gamers. A mod may make a niche change which is not to the taste of all players, and even the same player may wish to play the game in more than one style which different mod combinations enable.

History
id Software began in the 1990's to separate the engine from the media and provide tools to facilitate modding in their games, due to the popularity of previous user-made mods.

Examples
Minetest Game is an example of a Free game which is very much built around mods (as are many games made in Minetest). The game is designed around a collection of small mods which can be individually enabled and disabled (though that might remove basic functionality). The user (or more properly, the server admin) selects which of the numerous other mods (e.g. enemies, armor, wildlife) to include to make up a complete game experience.

Some other games which have many mods or extensive mod support include:
 * Oolite
 * Mindustry has a scraper to automatically find mods.