It's a lot simpler than any of the above.
In this context, civil is just an abbreviation for civilian. It's used because some or all of the fighters, on at least one side and sometimes both (or more than two sides, which has sometimes been the case in civil wars) are not soldiers, but civilians who hastily organize themselves into a fighting force to defend their cause. Here are some examples;
The American Revolution, although usually presented as a war between two nations, was actually a civil war in which some of the Americans formed volunteer militias (Tarleton's Legion, for example) to fight for the king and government against the rebels, while others hastily organized an army and numerous informal militias (like Ethan Allen's Green Mountain Boys in the part of New York that later split off to become Vermont) to fight against the king. It is seldom mentioned that when the fighting began, it had not yet been decided that the aim was to seperate from England entirely; some thought they were fighting to force the government in London to recognize their rights, and that after the fighting ended they would still be British citizens. It was only a couple of months later that the hastily improvised Continental Congress issued the Declaration Of Independance, making it a war to become a new country.
The Spanish Civil War involved two sides composed of passionate civilian volunteers. The Republicans, who were the left wing/socialist side, had actually won control of the government in an election. The armed forces refused to support the new government, and a large majority of them, including a very large percentage of the officers, left to form what they called the Loyalist side and rebelled, forming the right wing/fascist armies that eventually won. On both sides, civilian volunteers greatly outnumbered the professional soldiers. The Loyalists were openly and massively supported by Nazi Germany and Fascist Italy. The Republicans could have had support from Britain, France and the USA, but made the tactical error of accepting the official support of the USSR, which caused the beginning movements to support them in the western democracies to die stillborn.
The American Civil War saw the US armed forces split between both sides, with a majority (but not a huge majority) of the officers going with the South.
The armed forces were so small to begin with that when both sides formed mass armies, the civilian volunteers on both sides outnumbered the experienced soldiers by something like 30 to 1 at the outset, growing to more than 100 to 1 by the end. Of course, by that time most of the civilians had become hardened combat veterans, but still regarded themselves as civilians temporarily under arms, not professional soldiers.
I've been told that some of us are Civil War Re-enactors, and they probably know much more in detail about the mostly civilian makeup of the armies on both sides than I do.
An example of a three sided civil war was the 'War Of The Three Henrys' in France in the 1580s.Three different claimants to the throne, all named Henry, each formed their own army to fight it out. That war was made more violent because it had a religious dimension; two of the claimants were Catholic while one was Protestant, and all sides feared that a victory by those of the other religion would mean persecution of their religion. At that time in history, such fears were very justified.
So, civil war means civilian war.
Mastertank1
We who play and dance are thought mad by they who hear no music.