A lot of people are saying it’s primarily a NY thing, so I’d just chime in to say we use it in PA as well, at least in the Philly area, to refer to the northern parts of the state.

Not much more to it than youre going far enough north to be out of your city’s metro area, but staying in the same state. In PA I’d say upstate probably starts around the Poconos. I think new Yorkers kind of tend to use it to refer to the rest of the state, we wouldn’t tend to do that here, Central and Western PA are different things than Upstate PA, although there is definitely some overlap and there’s not exactly clearly defined borders.

I don’t know how many other states use the same terminology, but I wouldn’t be surprised if it’s pretty common in other largish states with larger population densities in the southern part of the state and lower densities in the north (I don’t know off the top of my head which other states that would apply to, maybe it’s only PA and NY)