The most common meaning for civilian is simply someone who is not in the military. It can also refer to any object that is not military in origin — e.g., "civilian clothes" or "civilian life."
The meaning of civilian as a non military person is a relatively new one, dating from the early 19th century. Before that it referred to the code of law that governed non-military life. In fact for hundreds of years a civilian was somebody who was a judge or otherwise an expert on the law that applied outside military courts. Because it comes originally from a French word, remember to spell it with only one "l," even though it sounds like it should have two.