Short definition
Pouring wine from its bottle into a separate vessel to separate sediment and expose the wine to air.
Decanting serves two purposes. For older wines (15+ years), it separates the liquid from sediment that has settled in the bottle — pour slowly and stop when you see grit reaching the neck. For young, tannic reds, it exposes the wine to oxygen, softening harsh tannins and opening up aromas. A young Barolo benefits from an hour in a decanter. An old Burgundy might need only a few minutes.