An early-20th Century political compromise, between the "War of the Rebellion" (the official name used by the victorious north) and "The War Between the States" (which Southerners were lobbying for).
Both of these are accurrate & appropriate labels.
"Civil War" greatly misrepresents that conflict. It was most definitely not a Civil War. Yet, that is the label chosen.
I suggest this is/was not merely an imbecilic mistake. It is/was a strategic, purposeful decision designed to instill a most desired belief & attitude.
If the infliction of war on the South was so noble; why the need for this fundamental deception? (hint: it wasn't noble or just....it was shameful)