The Periphery refers to the outer rims of the Galactic Empire, including planets such as Anacreon and Santanni. Imperial control is weakest in the Periphery, as planet governors there frequently possess their own fleets. Santanni revolts during Forward the Foundation. About 50 years after the First Foundation is established on Terminus, the emperor will grant the periphery autonomy. This effectually removes them completely from Imperial control, making the dissolution of the Empire far more apparent than it had been before.
In the years preceding the fall of Trantor the periphery became any area outside the empire as this area became larger the empire became less and less great .
As the empire decreased in size and power the stagnation that would ultimately cause the collapse of the empire increased