||This article needs additional citations for verification. (December 2009)|
An expansion pack, expansion set, or supplement is an addition to an existing role-playing game, tabletop game or video game. These add-ons usually add new game areas, weapons, objects, and/or an extended storyline to a complete and already released game. While board game expansions are typically designed by the original creator, video game developers sometimes contract out development of the expansion pack to a third-party company, (see Hellfire for Diablo), it may choose to develop the expansion itself or it may do both (Ensemble Studios developed the real-time strategy game Age of Empires III and the first expansion called the WarChiefs itself, but contracted Big Huge Games for the second expansion pack, the Asian Dynasties). Board games and tabletop RPGs may have been marketing expansions since the 1970s, and video games have been releasing expansion packs since the 1980s, early examples being the Dragon Slayer games Xanadu Scenario II and Sorcerian.
The price of an expansion pack is usually much less than that of the original game. As expansion packs consist solely of additional content, most expansion packs require the original game in order to play. Games with many expansions often begin selling the original game with prior expansions, such as The Sims Deluxe Edition (The Sims with The Sims: Livin' Large). These bundles make the game more accessible to new players. When games reach the end of their lifespan, the publisher often releases a 'complete' or 'gold' collection which includes the game and all its subsequent expansions.
Stand-alone expansion packs 
Some expansion packs do not require the original game in order to use the new content, as is the case with Half-Life: Blue Shift or Sonic & Knuckles. In some cases, a stand-alone expansion such as Heroes of Might and Magic III: The Shadow of Death, or Dungeon Siege: Legends of Aranna includes the original game. Stand-alone expansions are preferred by game retailers since they require less shelf space and are generally easier to move since they don't carry the prerequisite of owning the original game. Often, enhanced games or anthologies released as "Game of the Year edition", "Director's Cut", etc. are examples of standalone expansions. In some standalone packs if someone does not have the original or previous expansion packs, then they can't use certain units or races contained in the other games in multiplayer.
Console game expansion packs 
Expansion packs are most commonly released for PC games, but are becoming increasingly prevalent for video game consoles, particularly due to the popularity of online console services such as Xbox Live and the PlayStation Network. The increasing number of multi-platform games has also led to the release of more expansion packs on consoles, especially stand-alone expansion packs (as described above). Command & Conquer 3: Kane's Wrath, for example, requires the original Command & Conquer 3: Tiberium Wars to play on the PC, but Xbox 360 versions of both the original Tiberium Wars and Kane's Wrath are available, neither of which require one another.
Grand Theft Auto: London, 1969 was the first expansion pack released for the PlayStation. The game required the player to insert the London disc, remove it, insert the original Grand Theft Auto disc, remove it, then insert the London disc again in order to play.