Auxiliaries (from Latin: auxilia = "supports") formed the standing non-citizen corps of the Roman army of the Principate
(30 BC–284 AD), alongside the citizen legions. By the 2nd century, the auxilia contained the same number of infantry as the
legions and in addition provided almost all the Roman army's cavalry and more specialised troops (especially light cavalry
and archers). The auxilia thus represented three-fifths of Rome's regular land forces at that time. Like their legionary
counterparts, auxiliary recruits were mostly volunteers, not conscripts.
Auxiliary troops were mainly recruited from the
peregrini, i.e. free provincial subjects of the Roman Empire who did not hold Roman citizenship and constituted the vast
majority of the empire's population in the 1st and 2nd centuries (ca. 90% in the early 1st century). Auxiliaries also
included some Roman citizens and probably barbarians (barbari, as the Romans called peoples located outside the Empire's borders).
This was in contrast to the legions, which admitted Roman citizens only.