Reduce

Technique Type: Heat

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.

Source: http://en.wikipedia.org/wiki/Reduction_(cooking)