Citizen means that your documents state that you are from a specific country For example, I am a citizen of the United States of America Civilian(s) is a term usually used by the military to refer to a non-military /non-hostile person or a person that doesn't pose a threat to a specific place. For example, The Avengers (the super hero movie/team) are responsible for a lot of civilian deaths.

A "citizen" legally belongs to a nation, and is entitled to protections and privileges such as the right to vote. A "civilian" is someone who is not in the armed forces. In the United States, almost all people serving in the armed forces are citizens. They are citizens before they join the armed forces, they remain citizens while they are in the armed forces, and they continue to be citizens when they leave the armed forces. They are civilians before they join the armed forces. They are members of the military while they are in the armed forces. They become civilians again when they leave the armed forces.

Citizen means you have the passport (documentation) from a certain country. Whilst civilian is more to show you are not part of the armed forces.