The term fortified is used to describe wines that have had additional alcohol added to them during their manufacturing, extra to that produced by alcoholic fermentation. Occasionally, during normal fermentation, a wine may also, naturally, reach a level of alcohol usually associated with fortification. The type and strength of alcohol varies, as does the time of fortification, but in most cases the alcohol has to be derived from grapes.
Fortified wines can vary from bone-dry through to very sweet. The actual sweetness (residual sugar) and perceived sweetness can be controlled, and is affected by many variable, for example, starting must weight (amount of suger in the must before fermentaton), the duration and speed of fermentaton, the level of fortification, the level of acidity, and even the ageing of the wines through evaportation and concentration of the wines' components.
Fortified wines are produced around the world, but some of the oldest and best known come from Portugal.