The Immortals were the elite 10,000-man infantry unit of the Achaemenid Persian army.
Herodotus described this formation as an exceptionally well-equipped guard corps that always maintained a strength of exactly 10,000. When a soldier died, became ill, or left the unit, another immediately replaced him, creating the impression that the number never changed.
The unit served as royal guards and heavy infantry, carrying spears, shields, bows, and daggers. It was associated with major campaigns, including Xerxes I’s invasion of Greece in 480 BCE.
“Immortals” is the conventional English name, based on a Greek account. Some scholars connect it with an Old Persian title meaning “companions,” so the literal translation may not have been the unit’s Persian name.