A dictionary definition isn't a fact, it's a description. It is a fact that the lexicography of that not very good online dictionary described a civilian in those terms."Civilian" is "a person who is not a member of the police or the armed forces" Definition taken from The Cambridge Dictionary. Agree with it or not. Many armed forces personnel past and present believe it's only them who can be not deemed civilians. Not true. And anyway, the police or armed services when off duty are treated in any operational incident as civilians, although the course the law takes after that might differ from non-police or non-service personnel. But that's another matter.
I do love a thread where people chuck in their opinions without checking facts. Jolly good fun.
The problems with that description is that it is inadequate and incorrect. A terrorist is not a civilian, despite meeting the definition you offered. And a policeman is a civilian despite their colloquial and harmful habit of pretending they are some sort of paramilitary force. A fact which I'd have hoped (possibly in vain) all police staff would have been made aware during their basic training.

