The name Voyager's crew gave to a narrow corridor through Borg territory that was empty of Borg ships, a gap later explained by the presence of Species 8472.
The Northwest Passage is a band of space running through the heart of Borg territory in the Delta Quadrant. A long-range probe launched by USS Voyager in 2373 shows it to be free of Borg vessels, although it is filled with intense gravimetric distortions.
In the two-part Voyager episode Scorpion, Janeway's crew names the corridor after the sea route once sought through Earth's Arctic and plans to slip through it rather than face thousands of cubes or settle in the Delta Quadrant.
The reason for the Borg's absence soon becomes clear. The distortions are quantum singularities through which Species 8472 is entering from fluidic space, and the passage is their beachhead. The discovery leads Janeway to propose her uneasy alliance with the Collective.