Terok Nor was the Cardassian ore-processing station in orbit of Bajor, run by Gul Dukat during the occupation, which was abandoned in 2369 and became Deep Space Nine.
Terok Nor was built by the Cardassians with Bajoran forced labour to refine uridium ore stripped from the planet below. Dukat, prefect of Bajor, governed from its operations centre, Quark opened his bar on its Promenade, and Odo was first employed there to investigate crimes among the Bajoran workers.
When the Cardassians withdrew in 2369 they wrecked what they could not take, and the Bajoran government invited Starfleet to administer the station under a new name. Its past is revisited in Necessary Evil, Things Past and Wrongs Darker Than Death or Night, in which Kira travels back to see her mother living there as Dukat's companion.
The Dominion and Dukat restored the old name when they held the station in 2373 and 2374. A sister station, Empok Nor, stood abandoned in the Trivas system.