Expansion pack

An Expansion pack is an a set collection of content and/or fixes to either the original base game, or a previously-released expansion installed for that game. In general, these require the base game to be installed, although another category, standalone expansion packs, do not require the base game to be installed.

Alternative content to expansions are "addons", "map packs" and "mission packs". Typically, expansion packs are 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.