The Imperium of Man, also occasionally called the Imperium of Mankind and simply the Imperium, is the galactic empire under which the majority of humanity is united. The founder and nominal ruler of the Imperium is the god-like Emperor of Mankind, the most powerful human psyker ever known.[3a]
The Imperium is the largest and most powerful political entity in the galaxy.[3a][65] An Imperial planet might be separated from its closest neighbour by hundreds or thousands of light years. As a stellar empire, the size of the Imperium cannot be measured in terms of contiguous territory, but only in the number of planetary systems in its control.[3a] While it is commonly stated that the Imperium is an empire of a million worlds,[3a][10a] the Imperium controls countless worlds.[10f] Some say a million,[10a] millions,[10b] a billion[55] or even billions in total.[10d] The overall number of planets has never been exactly determined, entire departments of the Administratum are devoted to cataloguing every controlled world, a task without end, due to the constantly changing nature of the domain. New planets are constantly discovered, conquered or colonised while old ones are lost.[10f][67] The Imperium also controls untold millions of stars.[10e]
The Imperium rules a million worlds is a common saying heard across its citizenry. A million worlds sounds numerous and strong. A dominating force. Easily understood by the egos of men. In truth, the Imperium is spread out thinly across billions of worlds, individual motes scattered across the vastness of the galaxy. Vulnerable to its many perilous threats.[57] The vast majority of the galaxy is unexplored, unknown, and dangerous. Large swathes of Wilderness Space seperate and fragment even explored and populated regions of Imperial space.[72a][72b] It would be more accurate to describe the Imperium's domains as tiny islands adrift in an enormous ocean. Any claim of control over the space between stars in laughable, and so the majority of the Imperium remains huddled around the fires of their stars.[72c]
Several aliens and forces - Chaos, Tyranids, Eldar, Dark Eldar, Orks, Tau, Necrons - challenge the supremacy of the Imperium. From within, the Imperium is threatened more insidiously by rebellion, mutation, rogue psykers, and subversive cults. Without the protection of the Imperium, mankind would fall prey to the countless perils that threaten it