What term describes a mixture of fat and flour that is used as a thickening agent in cooking?

The term that describes a mixture of fat and flour used as a thickening agent in cooking is "roux." A roux is typically made by cooking equal parts of fat, such as butter, and flour together over heat. This combination is essential in creating a base for many sauces and soups, allowing the mixture to thicken and develop flavors. It's a foundational technique in classical cooking, particularly in French cuisine. The cooking process helps to eliminate the raw taste of the flour and to create a smooth texture in the final dish. This thickening method is crucial when preparing dishes that require sauces or gravies with the desired consistency.