the Mason-Dixon Line
noun /ðə ˌmeɪsn ˈdɪksn laɪn/
/ðə ˌmeɪsn ˈdɪksn laɪn/
[singular]- the border between the US states of Maryland and Pennsylvania that is thought of as the dividing line between the south of the US and the north. In the past it formed the northern border of the states where slaves were owned.