A beer style is an informal agreement between a brewer and a drinker, expressed via a label, by which the former tells the latter roughly what sort of beer they are about to buy.
– From The World Atlas of Beer, second edition, 2016
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Nope. A better definition is, “Beer style is a term used to differentiate and categorize beers by factors such as colour, flavour, strength, ingredients, production method, recipe, history, or origin”. This method is much older (like, pre the good lord above older). If a brewer decided a stout was a stout because it was strong and dark, and another brewer decided a stout was low ABV and pale, no one would get it outside of their respective regions. CDAs/Black IPAs are a prime, new age example.