Residual sugar

The natural grape sugar that remains in wine because it has not been converted into alcohol during fermentation.

Although a wine may be quite dry, winemakers often leave a tiny amount of sugar (pretty much undetectable as “sweet” to the tongue) in the wine to make it seem rounder and more appealing (sweetness has a slight “fat” feeling to it.)

With wines such as riesling, which can have varying and detectable levels of sweetness (Spätlese, Kabinett, etc.), the winemakers have left a greater amount of sugar in the wine.

Residual sugar should not be confused with the sugar used in chaptalization.

(The Wine Bible, Karen MacNeil)