Mason-Dixon Line

American Heritage® Dictionary of the English Language, Fourth Edition

1. The boundary between Pennsylvania and Maryland, regarded as the division between free and slave states before the Civil War. It was established between 1763 and 1767 by the British surveyors Charles Mason (1730-1787) and Jeremiah Dixon (died 1777).

Wiktionary

1. n. The boundary line between Pennsylvania and Maryland, as run before the Revolution (1764-1767) by two English astronomers named Charles Mason and Jeremiah Dixon.
2. n. The boundary between the free and slave states at the time of the American Civil War.