Magister militum (Latin for "Master of the Soldiers", plural magistri militum) was a top-level military command used in the later Roman Empire, dating from the reign of Constantine. Used alone, the term referred to the senior military officer (equivalent to a war theatre commander, the emperor remaining the supreme commander) of the Empire. In Greek sources, the term is translated either as strategos or as stratelates.
Read more about Magistri Militum: Establishment and Development of The Office, List of magistri Militum, Later, Less Formal Use of The Term