A Sector is a measurement of space used by the Imperium. They are divisions of a Segmentum. The size of a sector varies according to local demands and stellar density, but are usually equivalent to a cube with sides 200 light years long. A naval Battlefleet of warships defend each sector.
Sectors are in turn divided into Subsectors, which range from ten to twenty light years in diameter. Subsectors, rather than being general blocks of space like sectors, are instead centered on densely populated star clusters, important worlds, or meeting points of warp trade routes.
The areas between sectors and subsectors are vast, in fact making up more of the Galaxy than the Imperium controls. They are often unexplored, uninhabited, alien empires, or inaccessible by the warp. These zones are known by various names including Wilderness Space or wilderness zones, inter sectors, forbidden zones, empty space, or frontier space.