The Periphery is a region of the galaxy under the control of the Galactic Empire. It is part of the Outer Reach. At least three planets lie within the Periphery: Terminus, where Hari Seldon and his followers are exiled to in order to establish their Foundation; and the neighbouring planets of Anacreon and Thespis.[1]
While the Periphery is under the reign of the Galactic Empire, its reign is only de jure (rule in name or as a matter of legal recognition only). Whereas, in fact, the Periphery has been physically ceded by the Galactic Empire and left to its own demise since the Empire lacks the resources to police or maintain its hold on its galactic territory. Additional territory is expected to continue to be ceded as humanity's regression and the coming darkness slowly encroaches toward Trantor over the millennia since Hari Seldon and the Foundation were exiled to Terminus.