A mixture of flour and fat, typically in equal parts. They are cooked together at low temperatures, forming a mixture that is used to thicken and flavor stews, soups, sauces, and gravies. A roux is fundamental to velouté sauces, bechamel sauces, and more.

If your roux primarily being used as a thickener, then you want a lighter or blond roux. An example would be the use of a roux when making mac and cheese. If you are wanting to use your roux primarily to flavor your food, like in a stew, then you want a red roux. Of course, there are colors in between these two extremes.

If you don’t know for certain which roux to use, then cooking traditions can provide a useful hint. For example, a roux made as part of cajun cuisine might be made with oil as opposed to butter. Vegetable oils tend to have a higher burning point, meaning you could actually end up with a red roux as opposed to something that is burnt. Meanwhile, French cuisine, which makes heavy use of butter, will be more likely to make use of a blond roux.

Though the term is French in origin (translating to red or redhead), plenty of cultures have made roux throughout their history. While the modern use of the term can be traced back to 1793 (according to Merriam Webster), people in the Ottoman Empire made roux as far back as the 15th century. Lastly, even though you may not see the term roux in a recipe, you may still end up making a roux.

Side note: When referring to equal parts, this refers to either equal parts by weight or equal parts by volume.