The Eastern states with predominantly-large poor Orthodox Ashkenazim (refujews migrating from Russia, hence the 'borscht' tomato soup).
Some argue that this name is giving humorously to the summer vacation resorts of up-state New York, but it has grown since to include most of the eastern states, too.
Borscht Belt, a.k.a. Borscht Circuit.
The Borscht Belt is a refernce to the has-been resort strip along Route 17 in New York State just below the Catskill Park. Rumor has it that plans are in place to rejuvinate the once-happening area.
All the great Jewish Comedians performed at the Borscht Belt -- it was a happening place.