The Mason-Dixon line is a demarcation line separating four U.S. states, forming part of the borders of Pennsylvania, Maryland, Delaware, and West Virginia. It was surveyed between 1763 and 1767 by two Englishmen, Charles Mason and Jeremiah Dixon, to define the long-disputed boundaries of the overlapping land grants of the Penns, proprietors of Pennsylvania, and the Calverts, proprietors of Maryland. The line was marked by stones every mile and "crownstones" every five miles, using stone shipped from England. The Maryland side says (M) and the Delaware and Pennsylvania sides say (P). Crownstones include the two coats-of-arms.

The Mason-Dixon line is most famous for its role in the history of slavery. During the Congressional debates leading up to the Missouri Compromise of 1820, the term "Mason-Dixon Line" was used to describe the boundary between slave territory and free territory. Today, the Mason-Dixon Line still serves figuratively as the political and social dividing line between the North and the South, although it does not extend west of the Ohio River.

In popular usage, the Mason-Dixon line symbolizes a cultural boundary between the North and the South (Dixie). While the term "Mason-Dixon line" was used occasionally in the decades following the survey, it came into popular use when the Missouri Compromise of 1820 named "Mason and Dixons line" as part of the boundary between slave territory and free territory.