Writing in 1977, David S. Walls noted, "'Appalachian'...has never become a symbol of self-identification for the vast majority of the region's people, for whom the community, county, state, and nation remain more important units of political identity."  This is the case, Walls argues, despite the success of "the social movement to obtain recognition for Appalachia as a problem area"  as led by the ARC. There seem to be many reasons for this phenomena of which I will not attempt to venture into at the moment. However, if the vast majority of people from the mountains do not self-identify themselves as Appalachian, then why bother with a so-called "definition" for the region? One reason for familiarizing oneself with a definition is that within the larger national discussion, Appalachia is normally spoken of with the understanding of being a particular region with particular boundaries. Watch national news outlets or read newspapers and news sites directed to a nation-wide audience and you will soon discover that when the eastern mountains are spoken of it is often under the banner of "Appalachia." Take, for example, the various discussions that arise on an almost weekly basis related to coal. One would be hard pressed to find a national article that fails to mention the Appalachian region's place (or lack thereof) within America's production of coal. So there is a type of helpful regional, self-awareness that develops when one understands that when the media says Appalachia, they are talking about you if you happen to live in the eastern mountains. And if the discussion concerns coal, they are more often than not speaking about the central Appalachian region (see below). This is just one reason why it is important to have at least a general familiarity with the ARC's definition of the Appalachian region. There are countless other reasons that could be listed, but at this point we will move toward a definition.
According to the ARC, the Appalachian region “is a 205,000-square-mile region that follows the spine of the Appalachian Mountains from southern New York to northern Mississippi. It includes all of West Virginia and parts of 12 other states: Alabama, Georgia, Kentucky, Maryland, Mississippi, New York, North Carolina, Ohio, Pennsylvania, South Carolina, Tennessee, and Virginia.”  As indicated, West Virginia is the only state that fully lies within the Appalachian region.
The Subregions of Appalachia
Moving forward from the broad to the particular, from the whole to the parts, the ARC has further broken down the Appalachian region into smaller sections called subregions. At this point, the subregions of Appalachia begin to show more similarities amongst one another. One Appalachian scholar, Richard Drake, explains what these subregions looked like during the initial years of ARC’s inception:
The ARC defined four, later three, different Appalachian sub-regions. Its ‘Northern Appalachia’ was basically industrial Pennsylvania; its ‘Southern Appalachia’ was north Georgia and surrounding areas sharing in the growth of greater Atlanta, Birmingham, and Chattanooga; and the ‘Highlands’ were those portions of the Smoky Mountains and eastern West Virginia that are spectacularly beautiful, thus most appealing for tourist development. The last sub-region, ‘Central Appalachia,’ was that part of the region with the most difficult problems, resulting primarily from its experience with the coal industry. 
Finally, in 2009, the ARC further revised the regional categories into five subregions. The ARC divided “the Region into smaller parts for greater analytical detail and by using current economic and transportation data.”  Furthermore, “This classification is used only for research purposes and not to allocate ARC funds.”  The map below illustrates the current subregions of Appalachia. They are the following: Northern (green), North Central (purple), Central (yellow), South Central (red), and Southern (blue).
 Walls, David S. "On the Naming of Appalachia" in An Appalachian Symposium: Essays Written in Honor of Cratis D. Williams, J.W. Williamson ed., (Boone: Appalachian State University Press, 1977), 71.
 Drake, Richard B. History of Appalachia, (Lexington: University Press of Kentucky, 2001), 175.