Mason and Dixon's line Meaning, Definition & Usage
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noun the boundary between Maryland and Pennsylvania; symbolic dividing line between North and South before the American Civil War
Mason and Dixon line; Mason-Dixon line.
WordNet
Definitions
also, the Mason-Dixon Line , the boundary line between Pennsylvania and Maryland, as run before the Revolution (1764-1767) by two English astronomers named Charles Mason and Jeremiah Dixon. In an extended sense, the line between the free and the slave States.