Mason-Dixon Line

Proper Noun

• The boundary line between Pennsylvania and Maryland, as run (1764-1767) by two English astronomers named Charles Mason and Jeremiah Dixon, that, before abolition, defined part of the northern boundary of states in which slavery was permitted.
• The boundary between the free and slave states at the time of the American Civil War. Also alluded to in situations where political or cultural properties of the free and slave states are contrasted.