In cooking, reduction is the process of thickening and intensifying the flavor of a liquid mixture such as a soup, sauce, wine, or juice by boiling. Reduction is performed by boiling liquid (whether stock, wine, whiskey, vinegar, or sauce mixture) rapidly and usually without a lid (enabling the vapor to escape more easily) until the volume desired is reached by evaporation. Different components of the liquid will evaporate at slightly different temperatures, and the goal of reduction is to drive away those with lowest points of evaporation. It thus can be seen as a form of distillation, capturing those components that have the highest boiling point.
While reduction does concentrate the flavors left in the pan, extended cooking can drive away volatile flavor compounds or even juice from the meal, leaving behind dryer, less juicy food without essential flavors.