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. It had a total of five add-on packs released:
 * Operation: Anchorage
 * The Pitt
 * Broken Steel
 * Point Lookout
 * Mothership Zeta

Fallout: New Vegas
Fallout: New Vegas also featured add-ons, with a total of four pre-order bonuses, four story add-ons and two items packs.
 * Dead Money
 * Honest Hearts
 * Old World Blues
 * Lonesome Road
 * Gun Runners' Arsenal
 * Courier's Stash

Fallout 4
Fallout 4 released six add-ons in total, half of which were story add-ons, while the other half primarily expanded on the workshop settlement feature.
 * Automatron
 * Wasteland Workshop
 * Far Harbor
 * Contraptions Workshop
 * Vault-Tec Workshop
 * Nuka-World

Fallout 76
Fallout 76 is expected to contain add-ons in the form of free expansions after the game's release.