Generic definition, not the legal one. And, more particularly, this is a very recent shift in the usage of the term. Traditionally, and legally, the term "Civilian" meant someone not serving in the Military.
"Civilian is a person not serving in military ... Example of a caselaw using the term: Selective Training and Service Act of 1940, 50 U.S.C.A. App., sections 301-318. not only prescribed procedures for the selection, training and service of individual citizens of the United States in the armed forces, but also made provisions for the return to their previous civilian pursuits.[ Hanebuth v. Patton, 115 Colo. 166, 171 (Colo. 1946)]"
Civilian Pursuits Law and Legal Definition | USLegal, Inc.
civilian
1 a person learned in the civil or Roman law.
2 as an adjective, pertaining to the civil or Roman law.
3 a person not in the armed forces.
Civilians
"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."
Customary IHL - Rule 5. Definition of Civilians