Civitas Imperialis is a term in High Gothic which means the rule of Imperial law. Its actual meaning is more specific and, as Imperial scholars have learned from the personal diaries of Warmaster Slaydo, the first Imperial Commander of the Sabbat Worlds Crusade, was a concept of particular importance to that great military leader of the Imperium.
At the foundation of the Imperium of Man in the late 30th Millennium, the Civitas Imperialis was laid down by the Emperor of Mankind Himself to "...guarantee the safety and assurance of any citizen of the Imperium of Man, wherever he or she travels or sets foot within the length and breadth of the Imperium."
The Civitas was therefore meant to be the hallmark of refined Imperial culture, the measure of its power and security, and it was precisely this, the qualitative perfection of human civilisation, as opposed to the basic rule of law, that Slaydo was determined to restore to the Sabbat Worlds.
In the context of the Sabbat Worlds Sector, the Civitas was suspended when the region was classified as "unstable/hazardous", was no longer considered to be under Imperial governance, and the court of the sector's Lord Governor was removed to the neighbouring sector. Before their conquest by the Imperium in the 35th Millennium, the Sabbat Worlds were considered beyond the bounds of the Civitas Imperialis and an extremely dangerous region of frontier space.
The term Civitas Imperialis is taken from the Latin words civitas, which means "state, citizenship, city-state" and imperiosus, which means "imperious, domineering, powerful."
- Sabbat Worlds Crusade (Background Book), pg. 13