In 1982, New York State started a "temporary" surcharge on corporations called the MTA Surcharge. I put it in quotes becuase originally, it was supposed to be for only two years to close a budget surplus, and it still exists today, with no sign of going away. It affects 12 counties in the MTA district (wherever Metro North and LIRR service riders). The 12 counties are the 5 boroughs, Nassau and Suffolk on L.I., Westchester, Rockland, Dutchess, Putnam and Orange.
I guess you can officially call anything north of that "upstate" however, Sullivan County is still fairly close. Let's call the number of downstate counties 13* with the rest being upstate. There may be some who consider upstate only north of Albany, but at that rate, there will be no more upstate New York.