Definition of the Mason-Dixon Line noun from the Oxford Advanced American Dictionary

 

the Mason-Dixon Line

 noun
noun
NAmE//ˌmeɪsn ˈdɪksn laɪn//
 
[singular]
 
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the border between the U.S. states of Maryland and Pennsylvania that is thought of as the dividing line between the south of the U.S. and the north. In the past it formed the northern border of the states where slaves were owned.
See the Oxford Advanced Learner's Dictionary entry: the Mason-Dixon Line