What is the name of the evil empire in the original 'Star Wars' trilogy?

The evil empire in the original Star Wars trilogy is the Galactic Empire.

The Galactic Empire is the authoritarian regime that rules much of the galaxy in Star Wars, The Empire Strikes Back, and Return of the Jedi. It emerged from the collapsing Galactic Republic after Chancellor Palpatine used the Clone Wars and political emergency to concentrate power and eventually declared himself Emperor.

In the original trilogy, the Empire is represented by enormous military power, including stormtroopers, Star Destroyers, Imperial officers, Darth Vader, and the Death Star. It fights the Rebel Alliance in the Galactic Civil War, which forms the central political conflict of the three films released in 1977, 1980, and 1983.

The Separatists and Trade Federation belong primarily to the prequel-era conflict, before the Empire’s formation. The First Order is a later successor movement in the sequel trilogy, rising from remnants of the fallen Empire. “The Empire” is the common short form, but “Galactic Empire” is its formal name.