Add-on

Add-ons or expansions (also called expansion packs) are content created for games that is released separately from the main game release. Downloadable content (DLC) is a specific form of add-on distributed through the internet.

Fallout 3
The first game in the Fallout series to feature downloadable content was Fallout 3. With a total of 5 add-on packs released:
 * Operation: Anchorage
 * The Pitt
 * Broken Steel
 * Point Lookout
 * Mothership Zeta

Fallout: New Vegas
Fallout: New Vegas also features add-ons, with a current total of 4 pre-order bonuses and three add-on packs released as of July 19, 2011, with one more (Lonesome Road, release date is September 20th, 2011 on all platforms) to come:
 * Dead Money
 * Honest Hearts
 * Old World Blues
 * Lonesome Road