A sauce thickener is an ingredient or technique used to increase the viscosity of sauces, gravies, and similar liquid preparations, giving them a smoother, more cohesive texture that clings better to food.
The most common are: roux, fat and flour cooked together, central to French and Cajun/Creole cooking; cornstarch, which gelatinizes when heated with liquid into a translucent, glossy thickened sauce, common in Asian and American cooking; arrowroot, a plant starch that gelatinizes similarly but leaves a clearer, shinier finish, often used in gluten-free and dairy-free recipes; egg yolks, which thicken custards, hollandaise, and other creamy sauces through their emulsifying proteins; beurre manié, equal parts softened butter and flour kneaded into a paste and whisked into a simmering liquid to thicken it without lumps; and reduction, simmering a liquid to evaporate water and concentrate flavor and thickness without adding a separate agent.
Roux and egg yolks give a rich, opulent mouthfeel; cornstarch and arrowroot give a clearer, glossier finish; beurre manié and reduction thicken without added starch or protein. Thickeners are generally added toward the end of cooking, since overcooking can make them gluey and undercooking leaves the sauce thin.