"battles among fellow citizens or within a community," from civil in a sense of "occurring among fellow citizens" attested from late 14c. in batayle ciuile "civil battle," etc. The exact phrase civil war is attested from late 15c. (the Latin phrase was bella civicus). An Old English word for it was ingewinn. Ancient Greek had polemos epidemios.
Early use typically was in reference to ancient Rome. Later, in England, to the struggle between Parliament and Charles I (1641-1651); in U.S., to the War of Secession (1861-1865), an application often decried as wholly inaccurate but in use (among other names) in the North during the war and boosted by the popular "Battles and Leaders of the Civil War" series published 1884-87 in "Century Magazine."
"The war between the States," which a good many Southerners prefer, is both bookish and inexact. "Civil war" is an utter misnomer. It was used and is still used by courteous people, the same people who are careful to say "Federal" and "Confederate." "War of the rebellion," which begs the very question at issue, has become the official designation of the struggle, but has found no acceptance with the vanquished. To this day no Southerner uses it except by way of quotation .... "The war of secession" is still used a good deal in foreign books, but it has no popular hold. "The war," without any further qualification, served the turn of Thucydides and Aristophanes for the Peloponnesian war. It will serve ours, let it be hoped, for some time to come. [Basil L. Gildersleeve, "The Creed of the Old South," 1915]