A massive, cube-shaped spacecraft that was first encountered on stardate 42761.3 (2365), near System J-25 in the Delta Quadrant, by the U.S.S. Enterprise NCC-1701-D, thanks to Q.
The Borg cube is one of the most formidable technologies known to the Federation. Its lack of identifiable function spaces — such as a bridge, engineering, or personnel quarters — reveals the hive-like Borg collective consciousness and its drones. It not only carries "smart" weaponry that adapts, but it also can analyze an opponent's weapons systems and adapt defenses within seconds.
On stardate 43989., another Borg cube — sent to assimilate Earth and its technology — traveled to the Alpha Quadrant using a transwarp conduit to make their journey and was briefly led by Locutus, the assimilated Picard.
A third cube tried to conquer Earth by altering the timeline in 2373, but was destroyed with concentrated weapons fire by a Starfleet armada. It ejected a smaller, speherical ship, a Borg spehere, which created a time vortex that allowed the Sphere to travel back in time in an effort to stop humanity's first contact with an alien species. This effort was also thwarted, by Picard, his crew, and the Enterprise-E.