Rule 5. Definition of Civilians
Rule 5. Civilians are persons who are not members of the armed forces. The civilian population comprises all persons who are civilians.
Summary
State practice establishes this rule as a norm of customary international law applicable in international armed conflicts. It also applies to non-international armed conflicts although practice is ambiguous as to whether members of armed opposition groups are considered members of armed forces or civilians.
International armed conflicts
The definition of civilians as persons who are not members of the armed forces is set forth in Article 50 of Additional Protocol I, to which no reservations have been made.[1]  It is also contained in numerous military manuals.[2]  It is reflected in reported practice.[3]  This practice includes that of States not, or not at the time, party to Additional Protocol I.[4] 
In its judgment in the Blaškić case in 2000, the International Criminal Tribunal for the Former Yugoslavia defined civilians as “persons who are not, or no longer, members of the armed forces”.[5] 
No official contrary practice was found. Some practice adds the condition that civilians are persons who do not participate in hostilities. This additional requirement merely reinforces the rule that a civilian who participates directly in hostilities loses protection against attack (see Rule 6). However, such a civilian does not thereby become a combatant entitled to prisoner-of-war status and, upon capture, may be tried under national law for the mere participation in the conflict, subject to fair trial guarantees (see Rule 100).
Exception
An exception to this rule is the levée en masse, whereby the inhabitants of a country which has not yet been occupied, on the approach of the enemy, spontaneously take up arms to resist the invading troops without having time to form themselves into an armed force. Such persons are considered combatants if they carry arms openly and respect the laws and customs of war (see commentary to Rule 106). This is a long-standing rule of customary international humanitarian law already recognized in the Lieber Code and the Brussels Declaration.[6]  It is codified in the Hague Regulations and the Third Geneva Convention.[7]  Although of limited current application, the levée en masse is still repeated in many military manuals, including very recent ones.[8] 
Non-international armed conflicts
The definition that "any person who is not a member of armed forces is considered to be a civilian" and that "the civilian population comprises all persons who are civilians" was included in the draft of Additional Protocol II.[9]  The first part of this definition was amended to read that "a civilian is anyone who is not a member of the armed forces or of an organized armed group" and both parts were adopted by consensus in Committee III of the Diplomatic Conference leading to the adoption of the Additional Protocols.[10]  However, this definition was dropped at the last moment of the conference as part of a package aimed at the adoption of a simplified text.[11]  As a result, Additional Protocol II does not contain a definition of civilians or the civilian population even though these terms are used in several provisions.[12]  It can be argued that the terms "dissident armed forces or other organized armed groups … under responsible command" in Article 1 of Additional Protocol II inferentially recognized the essential conditions of armed forces, as they apply in international armed conflict (see Rule 4), and that it follows that civilians are all persons who are not members of such forces or groups.[13]  Subsequent treaties, applicable to non-international armed conflicts, have similarly used the terms civilians and civilian population without defining them.[14] 
While State armed forces are not considered civilians, practice is not clear as to whether members of armed opposition groups are civilians subject to Rule 6 on loss of protection from attack in case of direct participation or whether members of such groups are liable to attack as such, independently of the operation of Rule 6. Although the military manual of Colombia defines the term civilians as “those who do not participate directly in military hostilities (internal conflict, international conflict)”,[15]  most manuals define civilians negatively with respect to combatants and armed forces and are silent on the status of members of armed opposition groups.

[1] Additional Protocol I, Article 50 (adopted by consensus) (cited in Vol. II, Ch. 1, § 705).
[2] See, e.g., the military manuals of Argentina (ibid., § 712), Australia (ibid., § 713), Benin (ibid., § 714), Cameroon (ibid., § 715), Canada (ibid., § 716), Colombia (ibid., § 717), Croatia (ibid., §§ 718–719), Dominican Republic (ibid., § 720), Ecuador (ibid., § 721), France (ibid., §§ 722–723), Hungary (ibid., § 724), Indonesia (ibid., § 725), Italy (ibid., § 727), Kenya (ibid., § 728), Madagascar (ibid., § 729), Netherlands (ibid., § 730), South Africa (ibid., § 731), Spain (ibid., § 732), Sweden (ibid., § 733), Togo (ibid., § 734), United Kingdom (ibid., § 735), United States (ibid., §§ 736–737) and Yugoslavia (ibid., § 738).
[3] See, e.g., the reported practice of Israel (ibid., § 726), Jordan (ibid., § 743), Rwanda (ibid., § 746) and Syria (ibid., § 747).
[4] See, e.g., the practice of France (ibid., § 722), Indonesia (ibid., § 725), Israel (ibid., § 726), Kenya (ibid., § 728), United Kingdom (ibid., § 735) and United States (ibid., §§ 736–737).
[5] ICTY, Blaškić case, Judgment (ibid., § 751).
[6] Lieber Code, Articles 49 and 51; Brussels Declaration, Article 10.
[7] Hague Regulations, Article 2; Third Geneva Convention, Article 4(A)(6).
[8] See, e.g., the military manuals of Benin (cited in Vol. II, Ch. 1, § 714), Cameroon (ibid., § 715), Canada (ibid., § 764), Kenya (ibid., § 728), Madagascar (ibid., § 729), South Africa (ibid., § 731) and Togo (ibid., § 734).
[9] Draft Additional Protocol II submitted by the ICRC to the Diplomatic Conference leading to the adoption of the Additional Protocols, Article 25 (ibid., § 706).
[10] Draft Additional Protocol II, Article 25 as adopted by Committee III (ibid., § 706).
[11] See ibid., § 706.
[12] Additional Protocol II, Articles 13–15 and 17–18.
[13] Michael Bothe, Karl Joseph Partsch, Waldemar A. Solf (eds.), New Rules for Victims of Armed Conflicts, Martinus Nijhoff, The Hague, 1982, p. 672.
[14] See, e.g., Amended Protocol II to the CCW, Article 3(7)–(11); Protocol III to the CCW, Article 2; Ottawa Convention, preamble; ICC Statute, Article 8(2)(e)(i), (iii) and (viii).
[15] Colombia, Instructors' Manual (ibid., § 717).