For the community's dictionary

An Expansion Pack is an optional add-on that you can purchase to add additional content to your The Sims-game and expand it with new features. It usually includes a lot of new game mechanics, clothes, hair, furniture and often a new world or neighborhood for your sims to live in and/or explore. It can also include features such as new life states (vampires, werewolves, witches, fairies, aliens and so on), seasons, weather and pets. 

Expansion Packs were introduced in The Sims 1 and since then, every game in the main series have had several expansions released for it. 

For a list of the expansions released for the different games, please see the comments beneath this post. 

Please note that you need the base game in order to be able to play one or several of the expansions in a series