Mousseline is a light sauce or filling made by folding whipped cream or whipped egg whites into a butter-, cheese-, or purée-based mixture, giving it an airy, foam-like texture; the name comes from the French mousse, meaning "foam."
It is a classic French technique, most often used as a light topping for poached fish or crab cakes, as a filling for vol-au-vents and savory tarts, or, in sweetened form, as a filling for cakes, tarts, and pastries such as éclairs.
Mousseline differs from a plain mousse mainly in use: mousseline usually functions as a sauce or filling component within a larger dish, while mousse is more often served as a dessert or course on its own. Because the whipped component can deflate easily, mousseline is folded gently and kept chilled until served.