The Collective was a group mind. It was located at the Sanmarus Institute. Beings from several civilisations were part of the Collective. Its purpose was to form an intellect that could reach across space.
The Eighth Doctor, Charley Pollard and C'rizz visited the Collective and met Quyai, one of the beings that was part of the Collective. Quyai helped the Doctor thwart Darrakhaan's theft of the space-time navigator. (PROSE: Before Midnight)