Short definition
Exposing wine to air to soften tannins and release aromas, either by decanting, swirling, or using an aerator device.
Air is both friend and enemy to wine. A young Cabernet Sauvignon that tastes tight and closed will relax after 30 minutes of breathing. Swirling your glass is the simplest form of aeration. Pouring into a decanter is more aggressive. Aerator gadgets that attach to the bottle pour wine through a screen for instant effect. But go easy with aged wines — too much air can strip away the fragile aromas that make them special.