Roux is a cooked flour-butter mixture used to thicken many sauces. It is made by whisking flour into equal amount of melted or clarified butter, then cooking the mixture until a desired colour is achieved: white, blond or brown.

White roux  —  roux blanc  —  is cooked just long enough to lose its floury taste. It is used to thicken white sauces.

Blond roux  —  roux blond  —  is cooked until golden in colour. It is used to thicken white sauces.

Brown roux  —  roux brun  —  is cooked until light brown in colour. It is used to thicken brown sauces.