ala: (1) Division of Allied troops roughly equivalent in size to a legion (third to second century BC). One such unit supported each legion.
(2) A unit of auxiliary cavalry of similar size to an infantry cohort in the army of the Principate (late C1st–4th AD).
aquilifer: The standard-bearer who carried the legion’s standard (aquila), a silver, later gold, statuette of an eagle (C1st BC–3rd AD).
auctoritas: The prestige and influence of a Roman senator. Auctoritas was greatly boosted by military achievements.
auxilia (auxiliaries): The non-citizen soldiers recruited into the army during the Late Republican and Imperial periods. By the third century AD the difference between these and the citizen legions appears to have been minimal.
ballista: A two-armed torsion catapult capable of firing bolts or stones with considerable accuracy. These were built in various sizes and most often used in sieges (C3rd BC–6th AD).
bucellarii: Soldiers paid and supported by a particular commander and forming part of his household. These men were still part of the regular army and supposed to be loyal to the emperor. The name derives from the ration hard-tack biscuit (bucellatum) and emphasized the commander’s obligation to feed his soldiers (late C4th–6th AD).
cataphract: Heavily armoured cavalryman often riding an armoured horse. The Romans first encountered such warriors in eastern armies, but later made use of them themselves.
centurion: Important grade of officers in the Roman army for most of its history, centurions originally commanded a century of sixty to eighty men. The most senior centurion of a legion was the primus pilus, a post of enormous status held only for a single year (C4th BC–3rd AD).
century (centuria): The basic sub-unit of the Roman army, the century was commanded by a centurion and usually consisted of sixty, later eighty men (late C4th BC–3rd AD).
carroballista: A version of the scorpion mounted on a mule-drawn cart to increase mobility (C1st BC–6th AD).
cohort (cohors): By the first century BC the cohort replaced the maniple as the basic tactical unit of the legion. Auxiliary infantry were also formed into cohorts. Usually these consisted of six centuries of eighty soldiers with a total strength of 480 (C1st BC–3rdAD).
comes: Officers of the later Roman army, ranking below the Magistri Militum (late C3rd–6th AD).
comitatenses: Units included in the regional forces not tied to specific frontier provinces (C4th–6th AD).
commilito (pl. commilitones): Comrade: this familiar form of address was often employed by a Roman general when speaking to his troops, especially at times of civil war.
consul: The year’s two consuls were the senior elected magistrates of the Roman Republic, and held command in important campaigns. Sometimes the Senate extended their power after their year of office, in which case they were known as proconsuls.
decurion: Cavalry officer who originally commanded ten men. Under the Principate the decurion led a turma of about thirty horsemen (C1st–3rd AD).
dictator: In times of extreme crisis a dictator was appointed for a six-month period during which he exercised supreme civil and military power. Later victors in civil wars, such as Sulla and Julius Caesar, used the title as a basis for more permanent power (C5th–1stBC).
dux: Officers of later Roman army (late C3rd–6th AD).
dux (duces) limitis: Commanders of all troops (limitanei) within one of the regions into which the frontier provinces of the later empire was divided (late C3rd–6th AD).
equites singulares: The term used for the bodyguard cavalry attached to the staff of provincial governors under the Principate. These units seem to have been about 500 strong and were recruited from men seconded from the auxiliary alae (C1st–3rd AD).
equites singulares augusti: The emperor’s own horse guards for the first three centuries of the Principate, these provided an élite cavalry force to support the Praetorian Guard (C1st–3rd AD).
foederati: Allied barbarians obliged to provide military service to the emperor. Usually served in their own units and sometimes under their own commanders who normally held Roman rank (C4th–6th AD).
gladius: A Latin word meaning sword, gladius is conventionally used to describe the gladius hispaniensis, the short Spanish sword which was the standard Roman side arm until well into the third century AD. Made from high-quality steel, this weapon could be used for cutting, but was primarily intended for thrusting (C3rd BC–3rd AD).
hastatus (pl. hastati): The first line of heavy infantry in the Republican legion, recruited from younger men (late C4th–C2nd BC).
imaginifer: The standard-bearer who carried the imago, a standard bearing a bust of the emperor (C1st–3rd AD).
imperium: The power of military command held by magistrates and pro-magistrates during their term of office (C3rd BC–3rd AD).
legatus (pl. legati): A subordinate officer who held delegated imperium rather than exercising power in his own right. Legati were chosen by a magistrate rather than elected (C3rd–1st BC).
(1) legatus augusti pro praetore. This title was given to the governors of the military provinces under the Principate who commanded as representatives of the emperor (C1st–3rd AD).
(2) legatus legionis. The title given to legionary commanders under the Principate (C1st–3rd AD).
legion (legio): Originally a term meaning ‘levy’, the legions became the main unit of the Roman army for much of its history. Under the Republic and Principate they were large, predominantly infantry, formations of c. 4,000–5,000 men, but by late antiquity most seem to have dwindled to a strength of about 1,000.
limitanei: The grade of troops commanded by the duces limitis, the military commanders of the various regions, usually on the frontier, into which the provinces of the later empire were divided (C4th–6th AD).
Magister Militum: Title given to the senior officers of the later imperial army (C4th–6th AD).
Magister Equitum: (1) Second in command to the Republican dictator, the Master of Horse traditionally commanded the cavalry, since the dictator was forbidden to ride a horse (C5th–1st BC).
(2) Title given to senior officers of the Later Imperial army, equal in status to Magistri Peditum (C4th–6th AD).
Magister Peditum: Title given to senior officers of the Later Imperial army (C4th–6th AD).
maniple (manipulus): The basic tactical unit of the Republican legion, the maniple consisted of two centuries (late C4th–2nd BC).
ovatio (ovation): A lesser form of the triumph. In an ovation the general rode through the city on horseback rather than in a chariot (C5th BC–1st AD).
palatini: Units of higher status and prestige than the comitatenses, the palatini also formed part of the field armies of late antiquity (C4th–6th AD).
pilum (pl. pila): The heavy javelin which was the standard equipment of the Roman legionary for much of Rome’s history (C3rd BC–3rd AD).
praefectus castrorum: Third in command of a legion during the principate, this was an experienced officer who was usually a former primus pilus (C1st–3rd AD).
prefect (praefectus): Equestrian commander of an auxiliary cohort or ala (C1st–3rd AD).
praetor: Praetors were annually elected magistrates who under the Republic governed the less important provinces and fought Rome’s smaller wars.
Praetorian Guard: The military bodyguard of the emperors of the Principate, commanded by tribunes and the whole corps commanded by two Praetorian Prefects. They were disbanded by Constantine in 312 after supporting his rival Maxentius (C1st–4th AD).
princeps (pl. principes): The second line of heavy infantry in the Republican legion, recruited from men in the prime of life (late C4th–2nd BC).
quaestor: Magistrates whose duties were primarily financial, quaestors acted as deputies to consular governors and often held subordinate military commands (C3rd–1st BC).
quincunx: The chequerboard formation used by the Republican legion in which the three lines were deployed with wide intervals between the maniples, the gaps being covered by the maniples of the next line (late C4th–2nd BC).
scorpion: The light bolt-shooting ballista employed by the Roman army both in the field and in sieges. They possessed a long range, as well as great accuracy and the ability to penetrate any form of armour (C1st BC–6th AD).
signifer: The standard-bearer who carried the standard (signum) of the century (C3rd BC–3rd AD).
socii: The Italian allies of the Republic. After the Social War (90–88 BC) and the general extension of citizenship to most of the Italian peninsula the socii disappeared and all Italians were recruited into the legions (late C4th–C2nd BC).
spolia opima: The highest honour which a triumphing general could claim was the right to dedicate spolia opima in the Temple of Jupiter Optimus Maximus on the Capitol. The right could only be gained by killing the enemy general in single combat and was celebrated on only a handful of occasions.
testudo: The famous tortoise formation in which Roman legionaries overlapped their long shield to provide protection to the front, sides and overhead. It was most often used during assaults on fortifications (C3rd BC–3rd AD).
triarius (pl. triarii): The third and senior line of heavy infantry in the Republican legion, recruited from veteran soldiers (late C4th–2nd BC).
tribunus militum (military tribune): (1) Six military tribunes were elected or appointed to each Republican legion, one pair of these men holding command at any one time (C3rd–2nd or 1st BC).
(2) Under the Principate each legion had one senior, senatorial tribune and five equestrians (C1st–3rd AD).
tribune of the plebs (tribunicia potestas): Although holding a political office without direct military responsibilities, the ten tribunes of the plebs elected each year were able to legislate on any issue. During the later years of the Republic many ambitious generals, such as Marius and Pompey, enlisted the aid of the tribunate to secure important commands for themselves.
triumph: The great celebration granted by the Senate to a successful general took the form of a procession along the Sacra Via, the ceremonial main road of Rome, displaying the spoils and captives of his victory, and culminated in the ritual execution of the captured enemy leader. The commander rode in a chariot, dressed like the statues of Jupiter, a slave holding a laurel wreath of Victory over his head. The slave was supposed to whisper to the general, reminding him that he was mortal. Under the Principate only members of the imperial family received triumphs, but other commanders were granted the insignia of a triumph (ornamenta triumphalia) (C5th BC–4th AD).
turma: The basic sub-unit of the Roman cavalry for much of its history, the turma consisted of around thirty men. Under the principate it was commanded by a decurion (late C4th BC–3rd AD).
veles (pl. velites): The light infantry of the Republican legion, recruited from the poor or those too young to fight as heavy infantry. It is unclear whether they were identical to or superseded the rorarii, another term applied to light infantrymen in the Republican legion (late C4th–2nd BC).
vexillation (vexillatio): (1) A detachment operating independently, a vexillation might consist of anything from a few men to several thousand and could be drawn from several units (C1st–3rd AD).
(2) Many cavalry units of the later field armies were known as vexillations. They appear to have been similar in size to the old alae (C4th–6th AD).
vexillum: A square flag mounted crosswise on a pole, the vexillum was used to mark a general’s position and was also the standard carried by a detachment of troops (C1st–3rd AD). A general’s vexillum seems usually to have been red.