Roux

Roux is a mixture of flour and fat cooked together and used to thicken sauces. Roux is typically made from equal parts of flour and fat by weight. The flour is added to the melted fat or oil on the stove top, blended until smooth, and cooked to the desired level of brownness. A roux can be white, blond (darker), or brown. Butter, bacon drippings, or lard are commonly used fats. Roux is used as a t... (Wikipedia)