Sector 3556 is an area of space in the Delta Quadrant. The terminus of the Barzan wormhole shifted to Sector 3556 in the Delta Quadrant from a point beyond the Denkiri Arm in the Gamma Quadrant. It is not known where the terminus of the Barzan wormhole shifted after this incident.
In 2366, the Ferengi Arridor and Kol were stranded in Sector 3556 when they traveled through the wormhole but did not re-enter it before the terminus shifted again. (TNG: "The Price")