In histories of the American Civil War, the border states were those whose laws condoned slavery but that did not secede. They were Delaware, Kentucky, Maryland, Missouri and West Virginia (the District of Columbia is sometimes included for geographical convenience). West Virginia was formed in 1863 from the northwestern counties of Virginia that had seceded from Virginia after Virginia seceded from the Union. In the cases of Kentucky and Missouri, the states had two state governments during the American Civil War, one supporting the Confederacy and one supporting the Union.

In addition, two territories not yet states - specifically the Indian Territory (now the state of Oklahoma), and the New Mexico Territory (now the states of Arizona and New Mexico) also permitted slavery but did not leave the Union; yet very few slaves could actually be found in these territories despite the institution's legal status there. Oklahoma is often cited as a "border state" today, but Arizona and New Mexico are rarely if ever so characterized (interestingly enough, Arizona was the last state to recognize Martin Luther King's birthday as a public holiday).

With geographic, social, political and economic connections to both the North and South, the border states were critical to the outcome of the war and still delineate the cultural border that separates the North from the South. After Reconstruction, most of the border states adopted Jim Crow laws resembling those enacted in the South, but in recent decades some of them (most notably Delaware and Maryland) have become more Northern in their political and social orientation, while others (particularly Kentucky) have adopted a predominantly Southern persona.

Today, the phrase is also sometimes applied in common usage to the states of the upper South, which formed the northern tier of the Confederacy, such as Arkansas, Tennessee, Virginia, and North Carolina.