Fortified wine
From Fermented Beverage Production:
Fortified wines, also known as liqueur wines in Europe or dessert wines in the U.S, are produced by the fortification of fermented, partially fermented or unfermented grape must with wine-derived spirit. European Union regulations define Liqueur wines generally as those having an acquired alcohol content by volume of between 15% and 22%, and a total alcohol content (i.e. acquired alcohol plus potential alcohol) of at least 17.5% vol.; within these rules allowance is made for vino generoso, wines with 15.0% volume alcohol and less than 5 g l(-1) sugar produced in demarcated areas (Council Regulation (EC) No. 822/87, 1987).
Fortified wines must be distinguished from spirits made from wine. While both have increased alcohol content, spirits are the result of a process of distillation; while fortified wines have spirits added to them. Fortified wines generally have an alcohol content between that of wines and spirits.
One of the most important reasons for creating a fortified wine is that wine of less than 18% alcohol will spoil if stored in a breathable cask, such as a common oak barrel. Fortified wines of around 20% do not suffer this problem and were thus easier to age and ship, allowing vintners to serve a much wider market at a lower cost than shipping their wine in sealed bottles.