Roux

Roux is a cooking mixture of wheat flour and butter (or other fat). It is the basis of many types of French sauce for cooking.
Roux is the thickening agent of three of the mother sauces of classical French cuisine: "sauce béchamel", "sauce velouté" and "sauce espagnole". Clarified butter, vegetable oils, or lard are commonly used fats. It thickens gravy, other sauces, soups and stews. It is made from equal parts of flour and fat by weight. 
When used in Italian cooking, roux is traditionally equal parts of butter and flour. In Cajun cuisine, roux is almost always made with oil instead of butter and is dark brown in colour. This lends much richness of flavour, but less thickening power. Hungarian cuisine uses rendered lard or vegetable oil instead of butter for the preparation of "roux" (called "rántás" in Hungarian).