Most Session purists demand that the style be defined as any beer no higher than 3 or 4 percent ABV, just as those cask conditioned worker pints were back in the day. Today, most categorize 5% ABV (or less) a true session, while some brewers create Session Ales that are even higher. More @Wikipedia
Hover over any link to get a description of the article. Please note that search keywords are sometimes hidden within the full article and don't appear in the description or title.