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 or blonde, 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. It is important to note that it takes more roux to achieve the same thickness the darker the roux gets.