Expansion pack
From Styleguide
(Redirected from Add-on)
expansion pack
A piece of software that adds content such as new levels or characters to a previously-released game.
Expansion packs require a copy of the original game to run, as opposed to sequels, which do not require this. Italicize the name of an expansion pack as you would a game name.
Expansion acceptable after first reference.
The 4MB RAM expansion should be called the Expansion Pak on all references
Example: House Party, a popular expansion pack for The Sims, adds many new characters and objects to the game.
Wrong: add-on, add on, expander, expansion, pack, pak.
Also see: patch.
