A nebula lying between the Bajor and Dreon systems, mentioned in For the Cause when Sisko asks freighter captain Kasidy Yates about the route she takes around it.
The Rolor Nebula is a nebula in the Bajor sector, not far from the Demilitarised Zone between the Federation and Cardassia. It sits between the Bajoran system and the Dreon system, so cargo ships plying that route have to plot a course around it.
It is mentioned in the Deep Space Nine episode For the Cause, set in 2372. Odo and Eddington tell Sisko that they suspect Kasidy Yates of using her freighter, the Xhosa, to smuggle supplies to the Maquis. Sisko, unwilling to believe it, quietly sounds her out about her cargo runs, and the detour around the nebula comes up as he tries to account for her movements.
The nebula itself is never seen. It serves as one small detail in the episode's picture of Sisko torn between duty and his feelings for Yates.