According to that font of all knowledge, Wikipedia, a definition of 4.5% ABV or less has been proposed by beer writer Lew Bryson. I would heartily support that notion. In contrast, the Brewers Association has adopted a new session beer category within their Great American Beer Fest competition, with an alcohol content not to exceed 4.1% ABW (5.1% ABV).
My wife and I have been travelling the Pacific Northwest and beyond in search of all things beer for about 15 years. We are frequent visitors to Oregon, which is the past and current State Of Beer in our opinion, and many others'. But having been to most parts of Oregon, especially Portland (to excess), we were looking for new pastures. Recently we took a train trip on Amtrak to the one large town in the region that we had never before investigated properly: Eugene, Oregon.