The Unicomplex was the enormous Borg installation in the Delta Quadrant, built from thousands of linked structures, that housed the Borg Queen and served as the heart of the Collective.
The Unicomplex is first shown in the feature-length episode Dark Frontier. It sprawls across hundreds of kilometres of space, with cubes and other vessels constantly docking, and trillions of drones are said to live within it. At its core is Unimatrix 01 and the Queen's chamber.
In that story the Queen lures Seven of Nine back and tries to enlist her in a plan to assimilate Earth with a nanoprobe virus. Janeway follows in the Delta Flyer, fitted with a stolen transwarp coil and the Hansens' shielding designs, and takes Seven out.
The complex is seen again in Unimatrix Zero and in the finale Endgame, in which a pathogen carried by the future Admiral Janeway infects the Queen and the structure breaks apart.