A cortical array was a Borg device that was implanted into the brain during assimilation. It was critical to higher brain functions, such as language, and it contained an index of the drone's memory engrams.
In early 2376, Captain Rudolph Ransom ordered The Doctor to remove Seven of Nine's cortical array after he had deleted the EMH ethical subroutine. This would have given him the codes necessary to unlock the Equinox's enhanced warp drive, at the cost of Seven's mental functions. (VOY: "Equinox, Part II")
In her attempts to find the translink frequency for Unimatrix Zero in 2376, the Borg Queen dismantled several drones and examined their cortical arrays. When Seven of Nine was reconnected to Unimatrix Zero, she at first believed it was a malfunction of her cortical array. (VOY: "Unimatrix Zero")