See border states in Webster's New World College Dictionary
slave states bordering on the free states before the Civil War: Mo., Ky., Va., Md., & Del.
See border states in American Heritage Dictionary 4
The slave states of Delaware, Maryland, Virginia, Kentucky, and Missouri that were adjacent to the free states of the North during the Civil War. Virginia joined the Confederacy in 1861, causing its western counties to form the new state of West Virginia. It and other Border States remained tied to the Union despite strong Southern sympathies.