Wiktionary
illegal combatant
n. A person who engages in combat without meeting the requirements for a lawful belligerent according to the laws of war as specified in the http://en.wikipedia.org/wiki/Third%20Geneva%20Convention%20.
Usage examples of "illegal combatant".
If there is such a concept as legal combatancy, and rules which must be followed to attain that status, then failure to follow those rules places one in the implicit status of illegal combatant.