Besciamella is the Italian name for béchamel, a milk-based white sauce made by whisking hot milk into a butter-and-flour roux until smooth, seasoned simply with salt, white pepper, and often nutmeg. The French name is traditionally, if disputably, traced to Louis de Béchameil, a steward at the court of Louis XIV; the sauce's actual origins are murkier than the legend suggests.
It's a foundation for baked pasta dishes like lasagna and cannelloni, for gratins, and for creamy soups. Some cooks enrich it further with cheese such as Parmesan or Gruyère, or brighten it with herbs, spices, or lemon juice. Getting a smooth besciamella means cooking the roux gently and adding the hot milk gradually while whisking, until the sauce coats the back of a spoon.