What do you mean when you say that a wine is full-bodied or medium-bodied?

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Dear Dr. Vinny,
What do you mean when you say that a wine is full-bodied or medium-bodied?
—Rima, Kolkata, India
Dear Rima,
“Body” and “weight” are used somewhat interchangeably in wine-speak, referring to how heavy or viscous a wine feels in your mouth. It comes from a combination of alcohol, extract, tannins, glycerol, acidity and residual sugar (if any is left over after fermentation).
When talking about body, a simple example is to compare skim milk (light-bodied), whole milk (medium-bodied) and cream (full-bodied). Those are good baselines, but there are many more shades of body—like how adding milk or cream to your coffee or tea will make it a touch fuller. Or how coconut milk is heavier than coconut water, or pale ale is lighter-bodied than stout.
—Dr. Vinny