In the broadest sense, civilian is someone who is not a member of an armed force. Civilians may be individuals or groups such as political parties, organizations and corporations, religious communities, and unions. Civilians are also often the focus of international humanitarian law in times of armed conflict, especially when they are harmed by the actions of combatants.
The term is derived from the Latin word civilis, meaning “of the people.” The word is used in English from the early 19th century to refer to anyone who did not serve in the military or fight for their country. It was a relatively new concept, in contrast to soldiers who were always called the “military” class.
Generally, the term civilian is considered to apply to people who do not take part in hostilities and should be protected by international humanitarian law (see article 51 of the Additional Protocol to the 1949 Geneva Conventions). However, this principle is difficult to implement because it is easy for people who participate in a conflict to slip between categories of civilian and combatant. For example, in the movie Blackhawk Down, a woman who runs out to a fallen soldier to cry over his body is a civilian, but she becomes a combatant when she picks up his rifle and points it at the advancing US soldiers.
In practice, the distinction between civilians and combatants is more complex in internal armed conflicts. This is why the Additional Protocol to the Geneva Conventions and its commentaries establish that any individual who takes a direct part in hostilities, even only occasionally, loses the protection of international humanitarian law for the duration of that participation (API Arts. 45.1 and 51.3; APII Art. 13.3).
The term civilian is also used at the policymaking level to describe particular individuals occupying specified roles in the administration and guidance of, and budgeting for, the armed forces and defense enterprise. This group includes not only national security and defense officials, but also members of the Congress with their relevant committees, and a number of other institutions, most notably the private sector, universities, and think tanks. This civilian population is critical to the legitimate process of policymaking, but does not constitute a single profession like military officership or journalism. Rather, the civilians of the policymaking enterprise represent a set of skills that complements and guides that of professional military advice.