A Pilum is a long spear/javelin of Roman origin, typically measuring around two meters long and ending with a pyramidal iron head (about 60cm long, 7mm in diameter).
The Pilum was designed mainly for use as a throwing weapon (javelin), though it was also used effectively as a defensive weapon, especially when in close formation with other Pilum-carrying soldiers.
The Pilum was carried by Roman soldiers into battle, where it could be thrown as a ranged weapon, or held in a fixed position - in conjunction with a formation of shields - to ward off an enemy's charge. The latter such then-uncommon tactics would greatly contribute to making the Roman Republic (and later, the Roman Empire) one of the mightiest military forces of its era.
The Pilum hardly features in the series, despite it being the standard missile weapon within the Roman Legion. It only appears in Episode 1 of Season 2 "Fugitivus", carried by some of the milites in Glaber's command. All other Roman soldiers throughout the series appear to be carrying the (historically far less common) lancea, a far simpler and lighter form of javelin.
Historically, Glaber's force consisted of Praetorian militia, which may or may not have been equipped in the same manner as standard Legion troops. Hence, the use of the simpler lancea for militia may be acceptable from a historical point of view.
Users of this WeaponEdit
In the historical setting of the series - the Late Roman Republic - the pilum was the standard throwing weapon of the regular roman legionary. Usually, each miles carried two pila, one light and one heavy.
Other branches of the armed forces of Rome did usually not carry the pilum. For example, the velites (light skirmishing troops) carried several hastae velitares each (a light javelin). Other troops, such as auxiliary or militia infantry, carried the lancea, another type of javelin. These weapons were far less sophisticated than the highly effective pilum.