An unlawful combatant, illegal combatant or unprivileged combatant/belligerent is a civilian who directly engages in armed conflict in violation of the laws of war. An unlawful combatant may be detained or prosecuted under the domestic law of the detaining state for such action.
The Geneva Conventions apply in wars between two or more sovereign states. Article 5 of the Third Geneva Convention states that the status of a detainee may be determined by a "competent tribunal." Until such time, he is to be treated as a prisoner of war. After a "competent tribunal" has determined that an individual detainee is an unlawful combatant, the "detaining power" may choose to accord the detained unlawful combatant the rights and privileges of a prisoner of war as described in the Third Geneva Convention, but is not required to do so. An unlawful combatant who is not a national of a neutral State, and who is not a national of a co-belligerent State, retains rights and privileges under the Fourth Geneva Convention so that he must be "treated with humanity and, in case of trial, shall not be deprived of the rights of fair and regular trial."
While the concept of an unlawful combatant is included in the Third Geneva Convention, the phrase itself does not appear in the document. Article 4 of Third Geneva Convention does describe categories under which a person may be entitled to POW status; and there are other international treaties that deny lawful combatant status for mercenaries and children. In the United States, the Military Commissions Act of 2006 codified the legal definition of this term and invested the U.S. President with broad discretion to determine whether a person may be designated an unlawful enemy combatant under United States law. The assumption that such a category as unlawful combatant exists is not contradicted by the findings of the International Criminal Tribunal for the Former Yugoslavia in the Celebici Judgment. The judgment quoted the 1958 ICRC commentary on the Fourth Geneva Convention: Every person in enemy hands must be either a prisoner of war and, as such, be covered by the Third Convention; or a civilian covered by the Fourth Convention. Furthermore, "There is no intermediate status; nobody in enemy hands can be outside the law," because in the opinion of the ICRC, "If civilians directly engage in hostilities, they are considered 'unlawful' or 'unprivileged' combatants or belligerents (the treaties of humanitarian law do not expressly contain these terms). They may be prosecuted under the domestic law of the detaining state for such action".
The Geneva Conventions do not recognize any lawful status for combatants in conflicts not involving two or more nation states. A state in such a conflict is legally bound only to observe Article 3 of the Geneva Conventions and may ignore all the other Articles. But each one of them is completely free—and should be encouraged—to apply all or part of the remaining Articles of the Convention.
Read more about Unlawful Combatant: International Law and Practice, International Criticism of Unlawful Combatant Status