What is Civilian?

Legal Definition
In general, a civilian is "a person who is not a member of the military or of a police or firefighting force", as defined by Merriam Webster's Dictionary. From the perspective of the U.S. Department of Defense, Chapter 18 of Title 10 United States Code refers to non-military law enforcement officers as civilians since they are employees rather than enlisted personnel, and also in order to distinguish itself from military police. In military and law enforcement slang, the term "Civies" or "Civvies" are often used to refer civilian population or civilian clothing.

Under the laws of war (also known as international humanitarian law), a civilian is one not being a member of the armed services and does not take a direct part of hostilities in times of armed conflict. The term "civilian" is slightly different from a non-combatant under the laws of war, because some non-combatants are not civilians (for example, military chaplains attached to the belligerent armed forces or neutral military personnel). Under international law, civilians in the territories of a Party to an armed conflict are entitled to certain privileges under the customary laws of war and international treaties such as the Fourth Geneva Convention. The privileges that they enjoy under international law depends on whether the conflict is an internal one (a civil war) or an international one.
-- Wikipedia
Legal Definition
One who is skilled or versed in the civil law. A doctor, professor or student of the civil law. Also a private citizen, as distinguished from such as belong to the army and navy or (in England) the church.
-- Black's Law Dictionary
Legal Definition
A doctor, professor, or student of the civil law.
-- Bouviers Law Dictionary