A pseudo-food, usually liquid (or nearly so), put onto another food to enhance or hide that food's natural flavor. Sauces usually have strong flavors, and are never eaten by themselves (although applesauce is an exception). You may dip a food into a sauce, as with chips and salsa, or you might pour the sauce onto the food as with spaghetti and spaghetti sauce. Sauces may also be mixed into the food when cooking, as is often done with soy sauce.
Sauces include, but are not limited to: aioli sauce, applesauce, catchup, crab sauce, fondue, gravy, salsa, soy sauce, sweet and sour sauce, tomato sauce, and Worcestershire sauce. If you need more, cookery has set up an index of Sauce Recipes on E2.
The class of sauces usually does not include salad dressing and most table condiments.
Sauce is also internet slang for source, i.e., 'sauce?' means 'where did you get that?'