The vinculum is the central processing device of a Borg vessel, interlinking the minds of all drones aboard and keeping their thoughts in order.
The vinculum filters the individual thoughts of the drones on a ship, keeping them unified as part of the Collective. Its importance makes it both a strength and a weakness of Borg design.
In 2375 Voyager recovered a vinculum from a destroyed Borg vessel. It carried a virus planted by another species, which caused Seven of Nine to manifest the personalities of people the Borg had assimilated. The crew destroyed the device and freed her from its influence.