In the culinary arts, the term au sec (pronounced "oh-SECK") refers to a liquid that has been reduced until it is nearly dry. Indeed, au sec means "nearly dry" in French.
Reducing a liquid to au sec is a process most frequently seen in sauce making. It's often the acid component, such as wine or vinegar, that is reduced to au sec. Examples of sauces where the wine or vinegar are reduced to au sec include Béarnaise and beurre blanc.