magister militum

Magister militum (Latin for "master of soldiers"; pl.: magistri militum) was a top-level military command used in the late Roman Empire, dating from the reign of Constantine the Great. The term referred to the senior military officer (equivalent to a war theatre commander, the emperor remaining the supreme commander) of the empire. The office continued to exist and evolve during the early Byzantine Empire. In Greek sources, the term is translated either as strategos or as stratelates (although these terms were also used non-technically to refer to commanders of different ranks).