It is a French word meaning "pure", for the Community legislation it is a wine whose residual sugar content is below the 15g/l.
It is in the lower part of the ascending scale used to indicate sweetness, which starts with brut zero, brut nature, extra brut, brut sauvage (all these are highly dry), then brut (dry), extra dry (which, oddly enough, is sweeter than a brut), sec (slightly sweet), demi–sec (sweet enough) and doux (definitely sweet).
