An expansion (also known as expansion pack) is the term for a software package which is not a complete game in itself but adds supplemental content to an existing game. Alternate titles for expansions are "add-ons" and "mission packs. "Typically an option taken by a developer when they have ideas on how to improve a game that are too drastic to be a free patch, but are still similar enough to the original product that they don't have to release an entirely new title.
Expansions to MMO games are often seen as a dirty financial strategy, since players have already paid an initial box cost in addition to a monthly fee, however there is more at work in MMO games than with normal games. Without an influx of new players, MMO games die off, so companies need to keep fresh content on store shelves. By releasing expansions and subsequent compilations, they can maintain a presence in sale venues.