The "Northwest Passage" running through Borg Space
Borg space referred to the large area of space in the Delta Quadrant controlled by the Borg Collective. It was bordered by the Nekrit Expanse, preventing the Borg from encountering races such as the Ocampa. As of 2373 it consisted of thousands of star systems, although after their losses to Species 8472, the insurrection led by Axum, and the destruction of the unicomplex and the transwarp hub, the number may be lower. (VOY: "Scorpion", "Unimatrix Zero", "Endgame")
The current status of Borg space is unknown, but after the destruction of the unicomplex and a transwarp hub, it is likely to be in upheaval.