Roux

From New Italian Recipes:

Roux – A mixture of flour and butter [or sometimes oil] used to thicken sauces, soups, and gravies. Usually the butter is cooked with the flour in a heavy-bottomed pan over medium heat. Most roux are white roux, made by cooking the flour for only a minute or two. Brown roux—made by cooking the flour until pale brown to dark brown—is also used in many recipes, especially Cajun cooking.