The Alterans were an alien species from another galaxy in the Stargate Universe that evolved millions of years ago. They would eventually split into two factions: the Ancients/Lanteans, who sought scientific advancement, and the fervently religious Ori. The Ancients fled to the Milky Way galaxy, while the Ori remained in their home galaxy. Both groups ultimately ascended to energy form, leaving behind the traces of their respective empires.
Although they are human in appearance, they are much more advanced, and acted as the precursors of human life by seeding an evolution of their species in both the Milky Way and Pegasus Galaxy. Because of this they are alien and not categorized under current humankind.