Demi Sec literally translates as half dry. It's a medium sweet Champagne style and is increasingly popular. The amount of sugar (dosage) added after the second fermentation dictates the sweetness level of champagne. A Demi-sec champagne contains between 33-50 grams of sugar per litre.
The Demi-Sec style also has a historal connection in that 19th century consumer tastes as a whole, enjoyed very sweet wine. It's a style that pairs best with dessert.