Roux is flour and fat cooked together and used to thicken sauces. The fat is butter in French cuisine, but may be lard or vegetable oil in other cuisines. The roux is used in three of the mother sauces of classical French cooking: béchamel sauce, velouté sauce, and espagnole sauce. Clarified butter, vegetable oils, bacon drippings or lard are commonly used fats. It is used as a thickener for gravy, other sauces, soups and stews. It is typically made from equal parts of flour and fat by weight.
—Roux from Wikipedia, the free encyclopedia