Wounded in action
From Halopedia, the Halo wiki
Template:FOF-1 Wounded in Action, WIA, is a classification used by the United Nations Space Command for troops on the battlefield or in a report. It is used when a soldier receives wounds in battle worthy of remark.
When Spartans became missing or were killed they were classified as Missing In Action or wounded in action to give to give the illusion that Spartans never die. It also gave the troops more morale.