A Roman legion (from Latin legio "military levy, conscription," from legere - "to choose") normally indicates the basic ancient Roman army unit recruited specifically from Roman citizens. The organization of legions varied greatly over time but they were typically composed of perhaps 6,000 soldiers, divided into maniples and later into "cohorts". Maniples/Cohorts were divided into "centuries". In reference to the early Kingdom of Rome (as opposed to the republic or empire) "the legion" means the entire Roman army. For most of the Roman Imperial period, the legions were a part of the Imperial army and formed its elite heavy infantry, recruited exclusively from Roman citizens (provincials who aspired to the citizenship gained it when honorably discharged from the auxiliaries). A legion included a small cavalry attachment. The Roman army (for most of the Imperial period) consisted mostly of "auxiliary" cohorts who provided additional infantry, and the vast majority of the Roman army's cavalry. Because of the enormous military successes of the Roman Republic and the Roman Empire, the legion has long been regarded as the prime ancient model for military efficiency and ability.
Roman military personal equipment was produced in large numbers to established patterns and used in an established way. These standard patterns and uses were called the res militaris or disciplina. Its regular practice during the Roman Republic and Roman Empire led to military excellence and victory. The general word for army became exercitus, "exercise." Roman equipment (especially armor) gave them "a distinct advantage over their barbarian enemies." This did not imply that every Roman soldier had better equipment than the richer men among his opponents. According to Edward Luttwak, Roman equipment was not of a better quality than that used by the majority of its adversaries. Initially they used weapons based on Greek and Etruscan types. On encountering the Celts they based new varieties on Celtic equipment. To defeat the Carthaginians they constructed an entire fleet de novo based on the Carthaginian model. Once a weapon was adopted it became standard. The standard weapons varied somewhat during Rome's long history, but the equipment and its use were never individual. Vegetius, 4th century author of De Re Militari, describes the equipment he believed had been used by heavy and light infantry earlier in the empire. The names of some weapons have been changed from the Latin to the Greek forms and Greek names have been preferred, for unknown reasons, perhaps because the center of Roman military power had shifted from Rome to Constantinople. Vegetius says in translation:
The infantry (armatura) was heavy, because they had helmets (cassis), coats of mail (catafracta), greaves (ocrea), shields (scutum), larger swords (gladius maior), which they call broadswords (spatha), and some smaller, which they name half-broadswords (semispathium), five weighted darts (plumbata) placed in the shields, which they hurl at the beginning of the assault, then double throwables, a larger one with an iron point of nine ounces and a stock of five and one-half feet, which was called a pilum, but now is called a spiculum, in the use of which the soldiers were especially practiced, and with skill and courage could penetrate the shields of the infantry and the mail of the cavalry. The other smaller had five ounces of iron and a stock of three and one-half feet, and was called a vericulum but now is a verutum. The first line, of hastati, and the second, of principes, were composed of such arms. Behind them were the bearers (ferentarius) and the light infantry, whom now we say are the supporters and the infantry, shield-bearers (scutum) with darts (plumbata), swords (gladius) and , armed just as are nearly all soldiers today. There were likewise bowmen (sagittarius) with helmet (cassis), coat of mail (catafracta), sword (gladius), arrows (sagitta) and bow (arcus). There were slingers (funditor) who slung stones (lapis) in slings (funda) or cudgel-throwers (fustibalus). There were artillery-men (tragularius), who shot arrows from the manuballista and the arcuballista.
In the late Roman Republic and the early Roman Empire, most Roman infantry used swords (gladii) and specialized throwing spears (pila) as their main weapons. In the middle and Late Roman Empire, most Roman infantry used thrusting spears as their main weapons.