Butter is a legally-defined term in France, not just a word in the dictionary with subjective interpretations.
The first definition of butter in French law was in 1924, expanded later by the decree of 30 December 1988. 
The decree defined butter as a “water emulsion in fat content, obtained by physical processes, and whose components are of dairy origin.” Standard butter is considered to be 82% butterfat, 16% water, and 2% milk solids (such as casein protein and mineral salts.) If a butter differs from this, then the term “butter” must be followed be a qualifier (e.g. “light butter.”)
Some think the culturing that is done to the cream first gives French Butters more taste complexity. French chef’s abroad look down their nose at Anglo-Saxon butters (which usually only have 80% butterfat content.) Even just the 2% butterfat content does make a difference, just as there is a difference in taste between 1%, 2% and whole milk. A higher fat content also means less water, making it better for pastry doughs.
Some counter that most of French butter is as good as North American or British butter (and by inference, the reverse would be true.) But even they will admit that 10 to 20% of the brands available in France (depending on who you ask) really excel and are in a gourmet league all their own. In particular, butters such as Lescure and Échiré are often mentioned.
Small, artisanal American dairies are now beginning to produce cultured butters with fat contents of 86% and up. Commercial brands such as Plugrá match the 82% fat content of French butter.
The cream the butter is made will be pasteurised, unless it’s a butter such as “Beurre Cru.” The cream may be frozen first for transport or storage.
Butters from Normandy and Brittany tend to be softer wih more colour and aroma, because the cows in these regions eat grass year round. Butters from Charentes and the east of France tend to be harder, drier, paler and have a less distinct aroma.
Only New Zealanders top the French as butter eaters. The French eat about 18 1/4 pounds (8.3 kg) of butter each per year.
 Butter is also a protected term in the European Union overall, though the EU definition is a bit more relaxed: it allows butter to have a butterfat content between 80 and 90%.
Greenspan, Dorie. “Butter With a Pedigree. Ah, the French”. New York: The New York Times, 17 January 2001.