List of Roman army unit types

• Accensus – a reservist or light legionary soldier.
• Acceptarius – a discharged soldier.
• 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 – carried 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 trumpeteer 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 century.
• Clinicus – a medic.
• Cornicen – bugler
• Doctor – a trainer, subdivisions for everything from weapons to hornblowing
• Draconarius – Roman cavalry standard bearer
• Decurion – leads a troop of cavalry (14-30 men). Often confused with decanus.
• Decanus – leads a contubernium (a legionary tent group of 8 men)
• Discens – Miles in training for an immunis position.
• Dux – a general in charge of two or more legions. In the Third Century, 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, 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 – soldiers in the Roman army, who had served out their time and obtained their 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 of hastati. A high-ranking officer within a manipuli
• 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 – those soldiers of the military of ancient Rome who were "immune" from combat duty and fatigues through having a more specialist role within the army
• Legatus legionis – commander of a legion 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 their command
• Legionary – the heavy infantry that was the basic military unit 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 even 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 the 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 century in the first cohort with the exception of the first century of the cohort.
• Primus Pilus – (literally 'first file', not spear) the commanding centurion of the first cohort and the senior centurion of the entire Legion
• Princeps – Pre-Marian soldier, initially equipped with the Hasta spear but later adopted the Gladius, 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 manipulus 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 officer).
• Protectores Augusti Nostri (a.k.a. Protectors Divini Lateris) honorific title for senior officers singled out for their loyalty to the Emperor and soldierly qualities. Constitute an Order of Honour rather than a military unit. First appears in mid-Third Century 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 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 in the Roman army 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 – Allied troops from allied states in the Pre-Marian army before the Social war (BC 91-88)
• Speculatores and Exploratores – the scouts and reconnaissance element of the Roman army
• Supernumerarii – a kind of supernumerary soldier 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 century
• Tirones – a basic trainee.
• Triarius – Triarii were spearmen of the Pre-Marian armies equipped with the Hasta and 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 a 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 trumpeteer
• 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 – a standard-bearer (carried the Vexillum

Sub-units of the Roman legion

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 an officer called a decanus. When on the march a Legion would often march contubernium-abreast (8-abreast) and in the Imperial Legion, ten contubernia formed a centuria.
• Maniple (Manipulus) – a Maniple was the Pre-Marian sub-unit of the Roman Legions, consisting of 120 men (60 for the Triarii).
• Legio (Republic) – Legions in the Pre-Marian armies consisted of 60 maniples 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 legion 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

In the years following 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 non-commissioned officer called a decanus. Ten contubernia formed a centuria.
• Centuria – A centuria consisted of 80 men under the command of a Centurion and his Optio. Six centuriae formed a cohort.
• Cohors (Cohortes) – A cohort consisted of 480 men. The most senior ranking centurion of the six centuriae commanded the entire cohort.
• First Cohort (Cohors Prima) – The first cohort was a double strength cohort (consisting of five double-strength centuriae) with a size of 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 – These were temporary task forces composed of one or multiple centuriae detached from the legion for a specific purpose. Vexillations were under the command of an officer appointed by the Legatus.