Theme (Byzantine district)

The themes or thémata (Greek: θέματα, thémata, singular: θέμα, théma) were the main military and administrative divisions of the Byzantine Empire. They were established in the mid-7th century in the aftermath of the Slavic migrations to the Balkans and Muslim conquests of parts of Eastern Roman territory, and replaced the earlier ancient provincial system established by Diocletian and Constantine the Great.

Source: Wikipedia — Theme (Byzantine district) (CC BY-SA 4.0)