First expansion of the MMORPG Star Wars: The Old Republic. It's also the first expansion to raise the level cap of the game, from 50 to 55.
It's the only expansion in the game to feature story missions that are different between factions. The story in later major expansions are the very same, regardless if you are a Republic or an Imperial player.
In the time between this and the next major expansion, Shadow of Revan, there were two minor expansions (Galactic Starfighter and Galactic Stronholds) that added new features in the game.