I'm not sure if there is a text-book definition of "expedition" vehicle but I believe it would be something like this:
A 4x4 vehicle with large enough fuel capacity to venture on dirt roads and trails between fuel pumps, with decent comfort for the crew, a 2-way radio for normal communications and something like SPOT for emergencies, enough tools and spare parts (including tires) to get you moving if something breaks, with enough carrying capacity for extra fuel, water, shelter, food, and clothing for present and inclement weather.
Most likely it is a little different for everybody, depending on locale and laws.
As to where? I suppose that depends. It is entirely possible to cross vast tracts of the western United States without touching pavement. Certainly the same is true for other countries also.
Just 2 cents worth.