List of Roman army unit types

• Actuarius – A military or camp clerk.
• Adiutor – A camp or headquarters adjutant or assistant
• Aeneator – Military musician such as a bugler.
• Agrimensor – A surveyor.
• Aquilifer – Bearer of the legionary eagle.
• Alaris – A cavalryman serving in an ala.
• Architecti – An engineer or artillery constructor.
• Armicustos – A soldier tasked with the administration and supply of weapons and equipment. A quartermaster.
• Ballistarius – An artillery operator.
• Beneficiarius – A soldier performing an extraordinary task such as military policing or a special assignment.
• Bucinator – A trumpeter or bugler.
• Cacula – Servant or slave of a soldier.
• Capsarior – A medical orderly.
• Causarius – A soldier discharged for wounds or other medical reasons.
• Centurion – Officer rank, generally one per 80 soldiers, in charge of a centuria.
• Clinicus – A medic.
• Cornicen – A bugler.
• Doctor – A trainer, subdivisions for everything from weapons to horn blowing.
• Draconarius – Bearer of a cavalry standard.
• Decurion – Leader of a troop of cavalry. Often confused with decanus.
• Decanus – Leader of a contubernium.
• Discens – Miles in training for an immunis position.
• Dux – A general in charge of two or more legions. In the Third Century AD, an officer with a regional command transcending provincial boundaries, responsible directly to the emperor alone, usually appointed on a temporary basis in a grave emergency. In the fourth century AD, an officer in charge of a section of the frontier answering to the Magister Militum.
• Duumviri navales – Two men elected to equip and repair the Roman navy.
• Equites singulares Augusti – Elite cavalry unit tasked to guard the Roman Emperors. Usually commanded by a tribunus of praetorian rank.
• Evocatus – A soldier who had served out his time and obtained his discharge, but had voluntarily enlisted again at the invitation of the consul or other commander.
• Evocatus Augusti – Praetorian Guard who had served out his time and obtained his discharge, but had voluntarily enlisted again.
• Frumentarii – Officials of the Roman Empire during the 2nd and 3rd era. Often used as a Secret Service, mostly operating in uniform.
• Hastatus – The youngest of the heavy infantry in the pre-Marian armies, who were less well-equipped than the older Principes and Triarii. These formed the first line of battle in front of the Principes.
• Hastatus Prior – A centurion commanding a manipulus or centuria of hastati. A high-ranking officer within a manipulus or centuria.
• Hastatus Posterior – A deputy to the hastatus prior
• Hastiliarius – A weapons instructor.
• Imaginifer – A standard-bearer carrying the imago – the standard which bore a likeness of the emperor, and, at later dates, his family.
• Immunes – Soldiers who were "immune" from combat duty and fatigues through having a more specialist role within the army.
• Katepano – A Byzantine military rank or military official.
• Legatus legionis – A legion commander of senatorial rank; literally the "deputy" of the emperor, who was the titular commander-in-chief.
• Legatus pro praetore – Provincial governor of senatorial rank with multiple legions under his command.
• Legionary – The heavy infantry that was the basic military force of the ancient Roman army in the period of the late Roman Republic and the early Roman Empire.
• Magister militum - High ranked commander in the late Roman Empire. Equivalent of a general.
• Medicus – Physician or combat medic. Specializations included surgery, ophthalmology, and also veterinary. At least some held rank equivalent to a centurion.
• Megas doux – Commander-in-chief of the Byzantine navy.
• Miles or Miles Gregarius – The basic private level foot soldier.
• Numerus – A unit of barbarian allies not integrated into the regular army structure. Later, a unit of border forces.
• Optio – One per century as second-in-command to the centurion. Could also fill several other specialized roles on an ad hoc basis.
• Pedites – The infantry of the early army of the Roman kingdom. The majority of the army in this period.
• Peditatus – A term referring to any infantryman in the Roman Empire.
• Pilus Prior – Senior centurion of a cohort.
• Pilus Posterior – Deputy to the pilus prior.
• Praefectus Castrorum – Camp prefect, third-in-command of the legion, also responsible for maintaining the camp, equipment, and supplies. Usually a former primus pilus.
• Praefectus Cohortis - Commander of a cohort.
• Praefectus legionis agens vice legati – Equestrian officer given the command of a legion in the absence of a senatorial legatus. After the removal of senators from military command, the title of a legionary commander.
• Praetorians – A special force of bodyguards used by Roman Emperors.
• Primus Ordinis – The commanding officer of each centuria in the first cohort with the exception of the first centuria of the cohort.
• Primus Pilus – The centurion commanding the first cohort and the senior centurion of the entire Legion.
• Princeps – Pre-Marian soldier, initially equipped with the Hasta spear, but later with the pilum, these men formed the second line of battle behind the Hastati in the pre-Marian armies. They were also chieftains in Briton like Dumnorix of the Regneses.
• Princeps Prior – A centurion commanding a century of principes.
• Princeps Posterior – A deputy to the princeps prior.
• Principales – A group of ranks, including aquilifer, signifer, optio, and tesserarius. Similar to modern NCOs.
• Protectores Augusti Nostri – Honorific title for senior officers singled out for their loyalty to the Emperor and soldierly qualities. The protectores were an order of honor rather than a military unit. The order first appeared in the mid-200s AD.
• Quaestionarius – An interrogator or torturer.
• Retentus – A soldier kept in service after serving required term.
• Rorarii – The final line, or reserve, in the ancient pre-Marius Roman army. These were removed even before the Marian reforms, as the Triarii provided a very sturdy anchor.
• Sagittarii – Archers, including horse-riding auxiliary archers recruited mainly in North Africa, Balkans, and later the Eastern Empire.
• Salararius – A soldier enjoying special service conditions or hired as a mercenary.
• Scholae Palatinae – An elite troop of soldiers created by the Emperor Constantine the Great to provide personal protection of the Emperor and his immediate family.
• Scorpionarius – An artilleryman operating a scorpio artillery piece.
• Signifer – Standard bearer of the Roman Legion.
• Socii – Troops from allied states in the pre-Marian army before the Social War
• Speculatores and Exploratores – The scouts and reconnaissance element of the Roman army.
• Supernumerarii – Supernumerary soldiers who served to fill the places of those who were killed or disabled by their wounds.
• Strategos - General and military governor of a theme in the Byzantine Empire.
• Stratelates – A Greek translation for the rank of magister militum that was used in the late Roman and Byzantine armies.
• Stratopedarches – A term originally used to refer to a Roman camp prefect, it was later used for a Roman or Byzantine general or a Byzantine commander-in-chief.
• Tablifer – A standard-bearer for the guard cavalry.
• Tesserarius – Guard commander, one per centuria.
• Tirones – A basic trainee.
• Triarii – Spearmen of the pre-Marian armies, equipped with the Hasta, who formed the third line of battle behind the Principes.
• Tribuni angusticlavii – Military tribune of equestrian rank, five of whom were assigned to each legion.
• Tribunus militum - Officer in the Roman army who ranked below the legate but above the centurion.
• Tribunus militum laticlavius – Military tribune of senatorial rank. Second in command of a legion. Appointments to this rank seem to have ceased during the sole reign of Gallienus as part of a policy of excluding senators from military commands.
• Tubicen – A trumpeter.
• Urbanae – A special police force of Rome, created to counterbalance the Praetorians.
• Velites – A class of light infantry in the army of the Roman Republic.
• Venator – A hunter.
• Vexillarius – Bearer of a vexillum.

  Sub-units of the Roman legion

• Contubernium – The smallest organized unit of soldiers in the Roman Army. It was composed of eight legionaries led by a decanus. When on the march a Legion would often march contubernium-abreast. In the Imperial Legion, ten contubernia formed a centuria.
• Maniple – The pre-Marian sub-unit of the Roman Legions, consisting of 120 men.
• Legio – A legion in the pre-Marian armies consisted of 60 manipuli of infantry and 10 turmae of cavalry. By 250 BC, there would be four Legions, two commanded by each Consul: two Roman legions which would be accompanied by an additional two allied Legions of similar strength and structure. For every Roman Legion there would be an allied Legion.
• Turma – A unit of cavalry in the pre-Marian army, which usually consisted of 32 horsemen.

• Contubernium – The smallest organized unit of soldiers in the Roman Army. It was composed of eight legionaries led by a decanus. Ten contubernia formed a centuria.
• Centuria – 80 men under the command of a centurion and his optio. Six centuriae formed a cohors.
• Cohors – A cohors consisted of 480 men. The most senior ranking centurion of the six centuriae commanded the entire cohors.
• First Cohort – The first cohort was a double strength cohort, numbering 800 men. The centurion of its first centuria, the Primus Pilus, commanded the first cohort and was also the most senior centurion in the legion.
• Legio – A legion was composed of nine cohorts and one first cohort. The legion's overall commander was the legatus legionis, assisted by the praefectus castrorum and other senior officers.
• Vexillatio – A temporary task force of one or more centuriae detached from the legion for a specific purpose. A vexillatio was commanded by an officer appointed by the Legatus.