The title of "Imperial House" was given to the House whose family members were the Padishah Emperors or Empresses of the Known Universe. Whoever was the Imperial House took control of the Universe and sat upon the Golden Lion Throne.
For over 10,000 years, House Corrino was the ruling House of the Universe, giving it the title, "The Imperial House Corrino". For 81 generations, the Golden Lion Throne was passed down through the family until House Atreides took the Throne and defeated the Corrinos at the Battle of Arrakeen.
Only two Houses in the entire history of the Universe ever were Imperial: House Corrino and House Atreides.