A domesticus was a member of the protectores domestici, an elite guard unit of the Late Roman army, who served as bodyguards and staff officers to the emperor. As its name testifies, these were troops considered as belonging to the household of the emperor. After a few years' service in the corps, a domesticus would normally be granted a commission by the emperor and placed in command of a military regiment. The title was retained in the East Roman Empire, hellenized as domestikos, and acquired a variety of functions.
The head of the domestici, the comes domesticorum was a vir illustris. There were two of these comital commanders, for the horse - viz. foot units (comes domesticorum equitum vs. comes domesticorum peditum).