Before it became the Imperium, Tevinter was ruled by a dynasty of kings. And long before the Chantry there was a Circle of Magi: the society of mages in each city. The titles our modern Circles use—enchanter, senior enchanter, first enchanter—all originated here. But above the first enchanter, the Circles of Tevinter had another office: magister.
The magisters formed a council of the most powerful mages in the kingdom. They convened in Minrathous and held dominion over all magic in the land. When Darinius seized the throne in -1195 Ancient, the Court of the Magisters became the royal court, and "magister" was the only title of nobility recognized in Tevinter.
The Imperium today is a magocracy. Political power is solely in the hands of the magisters, who come only from the ranks of the Circle. Every young mage aspires to be a magister's apprentice because it's the best chance of ascending to the rank of magister themselves.
—From Black City, Black Divine: A Study of the Tevinter Imperium, by Sister Petrine, Chantry scholar