The west meant the Mississippi river for maybe a century, Missouri was the "west", and everything beyond that was referred to as "westward". There were no states past Missouri for nearly a century after our founding and we only had western States after the civil war and that changed and muddled everything.
In the beginning we had about 16 states considered east, on or near the coast, everything else up to the Mississippi was west, hence you have "mid-west". The term just rolled forward with expansion so that it sort of means everything up to Colorado.
I am personally more fond of 'belt' terminology. Rust belt, corn belt,bible belt, etc. Where do we put the dork belt?
Worry about the fact that the great leaders of industry,finance, politics, media, and general world influence are clustered in and around maybe six or so states and these people refer to the entire continental US beyond those few states as 'flyover state's.