Egg wash
An egg wash is beaten egg, sometimes loosened with water or milk, brushed onto pastry before baking to give the surface color, shine, and a better seal.
Whole egg gives balanced gold and shine. Yolk alone bakes darker and richer; white alone gives shine with little color; milk alone gives a soft matte finish.
Beyond looks, egg wash works as glue for sealing seams and as a light waterproofing layer on a base that has to sit under wet filling.