List of Roman army unit types

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This is a list of Roman army unit types.

  • Actarius – A military or camp clerk.
  • Adiutor – A camp or headquarters adjutant or assistant.
  • Aeneator – Military musician such as a bugler.
  • Agrimensor – A surveyor (a type of immunes).
  • 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 (a type of immunes).
  • 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 – Bugler.
  • Doctor – A trainer, subdivisions for everything from weapons to hornblowing.
  • Draconarius – Bearer of a cavalry standard.
  • Decurion – Leader of a troop of cavalry (14-30 men). Often confused with decanus.
  • Decanus – Leader of a contubernium (a legionary tent group of 8 men).
  • DiscensMiles 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.
  • 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 (missio), but had voluntarily enlisted again at the invitation of the consul or other commander.
  • 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.
  • 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 Roman Empire.
  • Medicus – Physician or combat medic. Specializations included surgery (medicus vulnerarius), ophthalmology (medicus ocularius), and also veterinary (medicus veterinarius). At least some held rank equivalent to a centurion.
  • 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. ("...agens vice legati, dropped in later Third Century")
  • 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 (literally 'first file', not spear) – 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 (he was killed by Gaius Salvius Liberalis' soldiers).
  • 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 (Non-commissioned officers).
  • Protectores Augusti Nostri (a.k.a. Protectores Divini Lateris) – 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 the Eastern Empire and Africa.
  • 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 (91–88 BC)
  • 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.
  • Tablifer – A guard cavalry standard-bearer
  • 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 militum angusticlavii or military tribune – Military tribune of equestrian rank, five of whom were assigned to each legion.
  • 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 (a type of immunes).
  • Vexillarius – Bearer of a vexillum (standard).

Sub-units of the Roman legion[edit]

Before the Marian reforms of 107 BC the structure of the legions was as follows:

  • 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 (8-abreast). In the Imperial Legion, ten contubernia formed a centuria.
  • Maniple (Manipulus) – The pre-Marian sub-unit of the Roman Legions, consisting of 120 men (60 for the Triarii).
  • Legio (Republic) – 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.

After the reforms of Gaius Marius, the organisation of the legions became standardised as follows:

  • 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 (cohort) – A cohors consisted of 480 men. The most senior ranking centurion of the six centuriae commanded the entire cohors.
  • First Cohort (Cohors Prima) – The first cohort was a double strength cohort (consisting of five double-strength centuriae), numbering 800 men (excluding officers). The centurion of its first centuria, the Primus Pilus, commanded the first cohort and was also the most senior centurion in the legion.
  • Legio (Imperial) – 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.