Given this, it is perhaps time we come up with a term to denote a specific beer having a significantly lower ABV than is normal for its style. This term shouldn't encroach on the both over- and misused term of "session" beer. While trying not get into the specifics that frequent the beer nerd diatribes in various blogs and forums, it is very easy to say that many, many US (and increasingly elsewhere) breweries are using the term session to mean that it doesn't get you blackout drunk by drinking one. However, this usage desecrates the historical interpretation of "session" and often describes beers that have nowhere remotely near a typical session ABV.
In order to appease both those adhering to the stringent definitions often used by our UK beer brethren but also placating those here in the American craft scene who, much like in the music world, need to describe everything well beyond the typical subgenres, I propose making a clear distinction between the terms "session" and "sessionable." "Session" should keep the more traditional meaning of lower ABV beers that historically are less that 4-4.5% (and you session fanatics can continue to decide where that line is, as that really isn't in the scope of this piece).
However, I propose that the term "sessionable" should now describe beers that are lower in alcohol than is usual for a given style and/or are easier on the palate. Sessionable would be the counterpart to our common usage of imperial. Just as we use imperial to describe a beer as a bigger, more robust version of its style, we can use sessionable to describe the beer as a smaller version of what is typical of its peers.
So how could we go about defining what is sessionable? If you want to get into the specifics of BJCP guidelines, perhaps the lower echelon of these guidelines could define sessionable. A beer may fit most of a style's guidelines but have a lower ABV/starting gravity. Take for example an American barleywine; if a beer fitting all of the other characteristics of a barleywine comes in at 6.6% ABV and low OG, it would help consumers to differentiate it from a much larger version to call that beer a sessionable barleywine.