The Cardassian space station that became Deep Space Nine was named Terok Nor.
Terok Nor was built by Cardassians in orbit around Bajor in 2346, using Bajoran slave labor during the Cardassian occupation. It served as a mining and refinery station and was commanded by Gul Dukat, the last Cardassian prefect of Bajor.
When the Cardassians withdrew in 2369, the Bajoran provisional government took control and requested Federation assistance. The Federation renamed the station Deep Space Nine and placed Commander Benjamin Sisko in charge. Soon afterward, the station was moved from Bajor to the mouth of the newly discovered Bajoran wormhole, making it a strategic gateway to the Gamma Quadrant.
A common mix-up is Empok Nor, another Cardassian station of the same Nor-class design. Empok Nor was abandoned elsewhere and later appeared in a Star Trek: Deep Space Nine episode, but it was never the station renamed Deep Space Nine. The station’s earlier name also returns in Mirror Universe stories, where “Terok Nor” refers to a parallel-universe counterpart.