The Oxford dictionary defines “civil war” as “a war between citizens of the same country”. The Union maintained that the USA was still one country therefore called it the Civil War. They won, we are still one country, so it’s properly called the Civil War. If the South had won it would be properly called the War of Southern Independence.
One more time. Civil War is a fight between two factions for control of the govt. The South was not fighting for control of the govt. They wanted to secede, be left alone and have their own government. The Union/Northern states invaded, subjugated, occupied and forced their govt on the South. You can thank Abe Lincoln for the police state government we live in today. He was responsible for destroying states rights.