The vast region of the Delta Quadrant controlled by the Borg Collective, containing thousands of systems and cubes, which the USS Voyager had to cross on its journey home.
Borg space is the heartland of the Collective, far from the Federation in the Delta Quadrant. Its existence is implied from the Borg's first appearance in The Next Generation, but it is Voyager that actually reaches it.
In Scorpion, at the end of 2373, Voyager arrives at its edge and finds the Borg losing a war against Species 8472. Captain Janeway strikes a bargain for safe passage, and the former drone Seven of Nine joins the crew as a result.
Later episodes return to Borg territory, including Dark Frontier, which shows the Unicomplex where the Borg Queen resides, and the finale Endgame, in which a transwarp hub inside a nebula is destroyed as Voyager uses it to reach Earth.