An expansion pack is an addition (extra part) of a video game, that adds to it. Expansion packs can add characters, worlds, weapons, or scenarios. They can be for all types of video games.
Most expansion packs need the main version of the game to play, although there are a few exceptions which can be purchased stand-alone, such as The Ultimate Doom.
Expansion packs are usually cheaper than the main game, or may even be free.
The term "Expansion Pack" was also used by Nintendo for a hardware add-on that plugged into the Nintendo 64 and was required for some games. It increased the RAM from 4 MB to 8 MB.