Chiliarch (from Greek: χιλίαρχος/χιλιάρχης, chiliarchos/chiliarches, meaning "commander of a thousand") is a military rank dating back to Antiquity.
The term was first used by Ancient Greek writers to render the Persian title hazarapatish, commander of 1,000 men in the Achaemenid Empire's army. In addition, the title was used for an officer who played an important role in the Achaemenid court, the commander of the 1,000-strong personal bodyguard of the Achaemenid kings, the melophoroi, drawn from the ranks of the Immortals. The latter often played a role analogous to that of a vizier in later times.
The latter office was adopted by Alexander the Great, first awarded to Hephaestion and after Hephaestion's death to Perdiccas. In addition, the commander of the Companion cavalry also bore the title of chiliarch, and the title was later adopted for commanders of a regiment formed of two pentakosiarchies of 512 men each.
The rank continued in use in later ages: Greek writers wrote of "chiliarchs" when referring to the Roman legionary tribunes, and in the Byzantine army, the title was used as an alternative to that of the droungarios and the taxiarches.
Later still, during the Greek Revolution, the title was given as a rank to significant kapetans, leaders of the irregular bands that made up most of the Greek rebels' army.