The pilum (plural pila) was a throwing spear commonly used by the Roman army in ancient times. It was generally two meters long, consisting of a one meter iron shank with pyramidal head socketed into a wooden shaft of similar length. A pilum usually weighed between two and four kilograms, with the versions produced during the Empire being a bit lighter. It had a diameter of about 7.5mm. Some versions of the weapon were weighted by an iron ball to increase penetrative power. Recent experiments have shown pila to have a range of 30 meters, although effective range of about half.
Legionaries of the Late Republic and Early Empire often carried two pila, with one sometimes being lighter than the other. Standard tactics called for a Roman soldier to throw his pilum (both if there was time) at the enemy just before charging to engage with his gladius. Some pila had small hand-guards, to protect the wielder if he intended to use it as a melee weapon, but it does not appear that this was a common tactic.
Most pila were constructed such that the iron shank would bend after it had penetrated a shield. In this way, it could not be easily removed and the enemy, if not killed by the pilum, would have to discard his now-unwieldy shield before going into combat. Early pila do not seem to have had this characteristic, and Gaius Marius is sometimes given credit for this modification.