A terrorist is a civilian, by definition. He's not in a recognised military organization, does not wear a uniform, or carry arms openly. Therefore he is civilian
Actually, per the original Geneva Conventions, he is an illegal enemy combatant, who can be regarded as a spy, saboteur, or whatever, and summarily shot. He is not regarded as a civilian, but as an ememy who is fighting illegally.
Due to poor history education, and corrupt judges, politicians, lawyers, and journalists, the American people have become very confused about this.