A domesticus was a member of the protectores domestici, an elite guard unit of the Late Roman army

Late Roman army

The Late Roman army is the term used to denote the military forces of the Roman Empire from the accession of Emperor Diocletian in 284 until the Empire's definitive division into Eastern and Western halves in 395. A few decades afterwards, the Western army disintegrated as the Western empire...

, who served as bodyguard

Bodyguard

A bodyguard is a type of security operative or government agent who protects a person—usually a famous, wealthy, or politically important figure—from assault, kidnapping, assassination, stalking, loss of confidential information, terrorist attack or other threats.Most important public figures such...

s and staff officers to the emperor

Roman Emperor

The Roman emperor was the ruler of the Roman State during the imperial period . The Romans had no single term for the office although at any given time, a given title was associated with 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

Domestikos

Domestikos , in English sometimes [the] Domestic, was a civil, ecclesiastic and military office in the late Roman Empire and the Byzantine Empire.-Military usage:...

, and acquired a variety of functions.

The head of the domestici, the comes domesticorum was a vir illustris

Vir illustris

The title vir illustris is used as a formal indication of standing in late antiquity to describe the highest ranks within the senates of Rome and Constantinople...

. There were two of these comital commanders, for the horse - viz. foot units (comes domesticorum equitum vs. comes domesticorum peditum).