In the culinary arts, the term egg wash refers to a mixture of beaten eggs and some sort of liquid which is brushed onto food, such as pastry, before baking.
Brushing egg wash onto pastry adds a golden color and sheen to the cooked pastry. If you've ever seen pastry or pie with a golden, shiny glaze on the surface, it was probably formed by brushing it with egg wash before baking.
Egg wash can also be used as a sort of glue to secure two edges of pastry together, as when making filled pastries, things like empanadas or other en croute recipes. This works because the protein in the egg coagulates when it's cooked, forming a stiff bond.
Egg wash can be used on other foods besides just pastry. Duchesse potatoes are usually brushed with egg wash before baking.
Egg Wash Recipe: The typical recipe consists of about two tablespoons of liquid for each egg. Using less liquid creates a darker egg wash. It's usually made with water, although milk or cream be used, too. You could also add a pinch of salt.