On the European continent, beers with the highest alcohol content are called triples. On the British Isles (and in the USA) the tradition is to call them Barley Wines. A beer is considered a triple in Belgium when the alcohol content is 9% alcohol by volume and higher. There are some commercial exceptions, where beers are labeled "triple" even they do not have the required alcohol strength.
Why do we call them triples? When the brewer starts with three times the ‘normal’ amount of malt in the brew kettle, more starches and sugars result from the boiling process and the end result is more alcohol after fermentation.