A Cortical node on a Borg drone
The Cortical node is a Borg implant that sits in a socket above the right eyebrow as a part of the Cortical array (VOY: "Human Error"). It is a vital implant to the Borg, it connects their mind with the collective. (VOY: "Unimatrix Zero, Part II")
It is also responsible for operating all the other implants, if it malfunctions they can fail and be rejected by the body. The technology in it is too advanced to attempt to repair, the Borg simply replace drones who suffer from malfunctions of the cortical node. (VOY: "Imperfection")
Built into every cortical node is a fail-safe mechanism designed to shut down the higher brain functions when the drone achieves a certain amount of emotional stimulation, to deactivate drones who start to regain their emotions. (VOY: "Human Error")
Nitpicks
In VOY: "Imperfection" the crew of Voyager replaces Seven of Nine's cortical node and the device is conveniently socketed, which seems to indicate easy replaceability instead or irreplaceability. This could be seen as an indication that Seven's understanding of Borg technology is imperfect, which seems to be unlikely, given her usual expertise.