Something that a game designer would use to create an adventure game, notably if it also happened to be a computer game. Engines are handy for dictating rulesets to make games emulate certain wargame or RPG systems. Most game engines are hoarded by their companies but a few have been sold or made available to the public, notably FRUA, Inform, AGT and the ilk. Open source games for Linux will no doubt change all of this.