The phalanx (Ancient Greek: φάλαγξ, Modern Greek: φάλαγγα, phālanga; plural phalanxes or phalanges; Ancient and Modern Greek: φάλαγγες, phālanges) is a rectangular mass military formation, usually composed entirely of heavy infantry armed with spears, pikes, sarissas, or similar weapons. The term is particularly (and originally) used to describe the use of this formation in Ancient Greek warfare, although the ancient Greek writers used it to also describe any massed infantry formation, regardless of its equipment, as does Arrian in his Array against the Allans when he refers to his legions. In Greek texts, the phalanx may be deployed for battle, on the march, even camped, thus describing the mass of infantry or cavalry that would deploy in line during battle. They used shields to block others from getting in. They marched forward as one entity, crushing opponents. The word phalanx is derived from the Greek word phalanx, meaning the finger.

While the Spartan phalanx used a shorter more versatile spear, the Macedonian phalanx that Alexander commanded used a "sarissa" which was a much longer and heavier spear which required the use of two hands.