The term Immortals comes from Herodotus who called them either Ten Thousand or Athanatoi (lit. immortals). Persians themselves probably did not use the term - Herodotus may have used a corruption of a term anusiya (companions).
Herodotus tells that Immortals were heavy infantry lead by Hydarnes that was kept constantly in strength of exactly 10.000 men - every killed, seriously wounded or sick member was immediately replaced with a new one. The regiment accepted only Medic or Persian applicants. Immortals were heavy infantry armed with a bow and a spear. The front rank would charge at the enemy when rear flanks shot arrows to support the assault.
The regiment's uniform consisted of a tiara or soft felt camp, an embroidered long-sleeved tunic, trousers and a coat of mail. Their weapons were a leather and wicker shield, a short spear with an iron point and a counterbalance on the other end, a bow with cane arrows, a quiver for them and a dagger or short sword.
The regiment was followed by a wagon train of covered carriages, camels and mules that transported their women and servants and they received special food.
Immortals participated in the battles of Marathon and Thermopylae and were in the Persian occupation troops in Greece in 479 BC under Mardonius. Alexander the Great defeated them in the Battle of Issus in 333 BC.
