The Galactic Padishah Empire encompasses the entire universe known to mankind. It was also commonly referred to as the Imperium, and more recently as the Old Imperium (heresy for some).
While the Padishah Emperor is seen as the Supreme ruler of the Known Universe, control of the Known Universe is shared, in a quasi-feudal arrangement, by the Landsraad, the Bene Gesserit, and the Spacing Guild.
For the first several millennia of its existence, the Golden Lion Throne of the Padishah Empire was occupied by House Corrino, though recently House Atreides has challenged this rule with the Jihad of Muad’Dib.
The Imperium referrs to the form of absolute trans-world governance used to maintain control of the Known Universe—it provided a socioeconomic and political framework within the Faufreluches class system.