Wounded in Action, better known as "WIA", is a classification used by the United Nations Space Command to denote service members that have been wounded during a battle situation. Being Wounded in Action will result in being rewarded with a Purple Heart.
When SPARTAN-IIs are killed, they are reported Missing in Action or Wounded in Action to create the illusion that Spartans never die. This was designed to help boost morale for the UNSC. It is extremely likely that all Spartans have been wounded in action at some point.