Theme (Byzantine district)
The themes or thémata (Greek: θέματα, thémata, singular: θέμα, théma) were the main military/administrative divisions of the middle Byzantine Empire. They were established in the mid-7th century in the aftermath of the Slavic invasion of the Balkans and Muslim conquests of parts of Byzantine territory, and replaced the earlier provincial system established by Diocletian and Constantine the Great. In their origin, the first themes were created from the areas of encampment of the field armies of the East Roman army, and their names corresponded to the military units that had existed in those areas. The theme system reached its apogee in the 9th and 10th centuries, as older themes were split up and the conquest of territory resulted in the creation of new ones. The original theme system underwent significant changes in the 11th and 12th centuries, but the term remained in use as a provincial and financial circumscription until the very end of the Empire.