I believe there are two definitions:
The Mason-Dixon line:
https://en.wikipedia.org/wiki/Mason%E2%80%93Dixon_line
This has Delaware in the North but the other states in the South.
Parallel 36°30′ north (the Missouri compromise line):
https://en.wikipedia.org/wiki/Parall...E2%80%B2_north
This has Delaware, Maryland, Virginia, West Virginia and Kentucky in the north with the other states in the south.