The general definition is East or West of the Mississippi, but, I crossed the Mississippi today after about 4 hours, then drove 9 *more* hours, and am still well short of halfway across, and have another 6+ hours or more before I even hit the front range of the Rockies - which is about the halfway point of my trip. And there are two more huge mountain ranges with snow on them after that.
Cheyenne, Wyoming, or thereabouts, is about the halfway point between Sunnyvale and Muncie (1200 miles), therefore I assert that anything east of Cheyenne is "eastern". That's also about the breakpoint between the Western terrain and the classic midwest Prairie.
At least one trip across and back is highly recommended for anyone, particularly easterners, because you would have a hard time grasping the scope of the endeavor any other way.
Brett
p.s. After having passed by again, I will officially define the dividing line between East and West (at least along i-80) as the ridge immediately prior to exit 323, Happy Jack Road exit, also the highest point on i-80 (8640 feet). To the east, it still looks like high plains, to the west, it's definitely the mountains and western scale. It's like you fall off the edge of a table as you go west, it has been a steady ramp up from 1000 feet at Omaha, Nebraska, to 8500+ but like a tilted pool table. Then, you crest the ridge, and you drop 1500 feet in 5 miles like you fell off the edge. Then it's more-or-less 7000ish feet +- 500 until you get to Park City Utah.
Note that on I-40, a similar spot is just east of Albuquerque, NM. On I-10, it's sort of out in the middle of nowhere east of Tombstone, Arizona. On 70 you only have a slight dropoff west of town but go south a bit, say, US-24, and the "edge" of the table is east of Colorado springs, and west of Schriever AFB. No one goes on I-70 or 24 any more, since the "World's Largest Prairie Dog" attraction in Oakley, Kansas, is no longer open.