What was the name of the elite infantry unit of the Persian army?

What was the name of the elite infantry unit of the Persian army? It was the Immortals, an elite corps of the Achaemenid army. The unit is chiefly known through the account of the Greek historian Herodotus.

Herodotus described the Immortals as a force of exactly 10,000 soldiers whose strength was kept constant. If a member died, became seriously ill, or was wounded, another soldier immediately replaced him. This replacement system likely inspired the impression that the unit’s members were “immortal,” although the Greek name may not precisely match the unit’s Persian designation.

The Immortals served both as elite heavy infantry and as a royal guard. They were mainly Persian, with Medes and Elamites also represented. Ancient descriptions associate them with spears, bows, swords or daggers, wicker shields, and scale armor.

They are often confused with the Spartans, who were Greek infantry, or with cataphracts, a later term for heavily armored cavalry. The Immortals belonged to the Persian imperial military system and were especially prominent in Greek accounts of the Persian Wars.