The Mason–Dixon line, also called the Mason and Dixon line or Mason's and Dixon's line, is a demarcation line separating four U.S. states, forming part of the borders of Pennsylvania, Maryland, Delaware, and West Virginia (part of Virginia until 1863). It was surveyed between 1763 and 1767 by Charles Mason and Jeremiah Dixon as part of the resolution of a border dispute involving Maryland, Pennsylvania, and Delaware in colonial America. The dispute had its origins almost a century earlier in the somewhat confusing proprietary grants by King Charles I to Lord Baltimore (Maryland) and by King Charles II to William Penn (Pennsylvania and Delaware).

The largest portion of the Mason–Dixon line, along the southern Pennsylvania border, later became informally known as the boundary between the Southern slave states and Northern free states. This usage came to prominence during the debate around the Missouri Compromise of 1820, when drawing boundaries between slave and free territory was an issue, and resurfaced during the American Civil War, with border states also coming into play. The Confederate States of America claimed the Virginia portion of the line as part of its northern border, although it never exercised meaningful control that far north – especially after West Virginia separated from Virginia and joined the Union as a separate state in 1863. It is still used today in the figurative sense of a line that separates the Northeast and South culturally, politically, and socially (see Dixie).