Orb of Memory at Kai Opaka's temple on Bajor.
Though not officially named on screen, the Orb of Memory was one of the nine orbs of the Bajoran people. It was the first orb that was shown, and it gave Benjamin Sisko and Jadzia Dax visions of their pasts, which they were capable of interacting with. These visions showed memories which were particularly important to them; Sisko seeing the first day he met Jennifer Sisko, and Dax seeing the day the Dax symbiont was transplanted from the dying Curzon Dax into her (DS9: "The Emissary.").
When Sisko visited Kai Opaka on Bajor, he was shocked when she handed him the orb to take with him after his orb experience. He even attempted to hand it back, but Opaka insisted he take it to the station. Once it was taken aboard the station, Dax had her orb experience while studying it. After her studies, the orb was never seen nor mentioned again in the series, so what happened to it is unknown (DS9: "The Emissary.").