Demi-Beurre is half-fat butter.
It’s a term used to describe a butter that is “light” in fat. By law though in France, the packaging must still give the actual butterfat content.
In France, the butterfat content of Demi-Beurre is 41% — which is exactly half the minimum butterfat allowed in butter in France, 82%. In practice, though, French law allows the butterfat content of Demi-Beurre to range from 39 to 41%.
The term is also used in Germany, though in German of course — “Halbfettbutter” (“half fat butter.”) In Germany, normal butter is 80% butterfat.
Demi-Beurre is usually only made commercially from pasteurized milk. It can be salted or unsalted. There may be other ingredients such as a starch, which will be shown on the label, but no other fats are allowed.
Some people like Demi-Beurre for the perceived health benefits; some like it because it spreads somewhat more easily.
Some say don’t heat Demi-Beurre at all; more practical souls will add that you can gently heat it, but don’t attempt to fry with it.