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 Alans 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 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 the Great and Phillip II commanded used a "sarissa" which was a much longer and heavier spear which required the use of two hands.
The term itself, as used today, does not refer to a distinctive military unit or division (e.g., the Roman legion or the contemporary Western-type battalion) but to the general formation of an army's troops. Thus a phalanx does not have a standard combat strength or composition but includes the total number of infantry, which is or will be deployed in action in a single phalanx formation.
Many spear-armed troops historically fought in what might be termed phalanx-like formations. The word has come into use in common English to describe "a group of people standing, or moving forward closely together"; c.f. "a phalanx of police".
This article, however, focuses on the use of the military phalanx formation in Ancient Greece, the Hellenistic world, and other ancient states heavily influenced by Greek civilization.