In the fictional universe of Warhammer 40,000, the Sisters of Battle, also known as the Adepta Sororitas, are female warriors belonging to the "Ecclesiarchy", the religious arm of the Imperial government. The Sisters devotedly serve the Emperor of Mankind. They enforce the Imperial religion, seeking out and destroying heretics that defy the Emperor's Will.
They were officially founded in 36th Millennium by the Ecclesiarch Alexis XXII as Ordes Militant. In the 2nd edition of the game they were a standalone army, but, like the Grey Knights, the 3rd edition codices saw them joined to an Inquisitorial force. They now comprise the centillion strong Chamber Militant of the Ordo Hereticus, "Witch Hunters", which is tasked with hunting down and destroying heretics and unsanctioned psychics (commonly known as "rogue psykers" or "witches").
Also they guard all of the shrines of the Imperium and are based on billions of additional sacred or shrine worlds, counted separately from the regular Imperial empire due to their forbidden status and the limitless number of saints and divine angels of past warriors and mammoth cathedral-castellum war vehicles ready to be unleashed via the activation of holy relics. These forces often come to be the salvation of Imperial forces who only see towering giants of holy light or a vengeful cloud of angels with the faces of those who died as heros. The shrine and sacred worlds also provide much of the blessed vehicles and weaponry used by the Inquisition and the Ecclesiarchy in planet wide temple-depots responsible for production, consecration, and storage. and As such, the Sisters are found within the army book Codex: Witch Hunters. The Sisters of Battle are also a playable race in the Dawn of War real-time strategy game, as part of the Soulstorm expansion.
Though not the same as the Space Marines, they do wear similar power armor and have the same general unit set-up. Their weapons tend to be heat- and flame-based, because of the 'purifying' effect of fire, inspired mainly by the popular portrayal of the Emperor as a cleansing fire. This creates an interesting situation as the sororitas lean towards two extremes of weapon types: melta weapons for anti-tank and flame weapons for anti-infantry, both being heat based weapons. They wear markings determined by their designation, which are Orders instead of Chapters.
Though their faith in the God-Emperor is unshakable and their skill at arms is far beyond that of most soldiers, the Sisters of Battle remain simply human. They lack the extensive biological enhancements of the Space Marines and cannot match the superhuman Astartes in terms of physical ability and endurance. However, they do have access to power armor and weapons which are well beyond those issued to ordinary human soldiers, and are able to utilize powerful faith-based abilities tuned to them genetically to help them combat their foes on a temporary superhuman level akin to living saints. These advantages make them a formidable opponent for any army.