Definition - What does Sweet mean?
The adjective "sweet" to describe a wine can mean two different things. One is used to describe a wine which is sweet due to certain concentrations of residual sugar left in the wine following fermentation. These may be dessert wines, such as late harvest wines. It also can mean a wine has a perceived sweetness due to other elements in the wine, such as higher alcohols.
WineFrog explains Sweet
Sugar content in wine can make a wine sweet. Grapes that are harvested with a high Brix level, i.e., 27+, will often have a fermentation which is cut short due to the yeasts' inability to digest high amounts of sugar. This stopped fermentation may stop naturally or occur per the desire of the winemaker. Therefore, the wine will have an amount of residual sugar, making it sweet.
On the other hand, a wine may also have a sweet character due its content and structure. For instance, a wine which has undergone a secondary (malolactic) fermentation will contain lactic acid. This acid on the palate is much "sweeter" in mouthfeel, compared to malic acid. Other higher alcohols which are created through the fermentation of sucrose, fructose and/or glucose can also alter the viscosity of a wine, making it "sweeter" without the presence of actual sugar.