Short definition
On a sparkling wine label, sec means medium-dry with 17–32 grams of sugar per liter — noticeably sweeter than Brut.
In still wine, "sec" simply means dry. But on a Champagne or sparkling wine label, "Sec" falls in the middle of the sweetness scale — sweeter than Brut and Extra Dry, drier than Demi-Sec. It is an older style that was popular before modern tastes shifted toward very dry Champagne. You will occasionally find it, usually paired with dessert courses or fruit-based dishes.