Missing in action
From Halopedia, the Halo wiki
Missing in Action, better known as MIA, is a casualty classification used by the United Nations Space Command that refers to members of the UNSC who has been reported missing following a combat mission and whose status as to injury, capture, or death is unknown. Missing combatants must not have been otherwise accounted for as either Killed in Action or a prisoner of war.
When SPARTAN-IIs are killed, they are reported as Missing in Action or Wounded in Action to create the illusion that Spartans never die. This helped boost morale in the UNSC. SPARTAN-IIIs are not normally subject to this, as the very existence of the program remains classified. However, at least two were listed as such in accordance with the tradition by Kurt Ambrose.