Dry vermouth is a fortified, aromatized white wine flavored with herbs, roots, and botanicals (wormwood being the defining bittering component). It’s drier, sharper, and more herbal than sweet vermouth, with less residual sugar.
In cocktails it adds brightness, herbal lift, and a wine-like acidity that makes spirits taste less flat. It’s essential in a classic Martini style and also tightens up many gin and whiskey drinks.